Add package dependency flow documentation

Documentation/package-dependency-flow.md

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+# Package Dependency Flow
+
+This document describes how package dependencies are handled within source build.
+It describes the mechanisms that exist to control which package versions are used.
+
+## Origins of Packages
+
+A source build must be self-contained, meaning the entire product must be built
+from source in an offline environment. To achieve this, all packages dependencies must
+be satisfied by one of the following:
+
+### Source-Build-Reference-Packages
+
+The first repo that is built as part of source build is
+[source-build-reference-packages](https://github.com/dotnet/source-build-reference-packages).
+This repo contains all of the reference packages used to build the product. The repo
+contains [tooling to generate new reference packages](https://github.com/dotnet/source-build-reference-packages?tab=readme-ov-file#adding-new-packages).
+
+### Current Source Built Packages
+
+This refers to all of the packages produced in the current build. The set of packages
+available to each repo varies based on its build order. For example the msbuild repo
+can take a dependency on the current version of Microsoft.CodeAnalysis from roslyn
+because roslyn builds before msbuild. Conversely, since roslyn builds before msbuild,
+roslyn cannot take a dependency on the current version of Microsoft.Build; it can only
+take a dependency on a previously released version.
+
+### Previous Source Built Packages
+
+Because the .NET product uses itself to build, the .NET source build product must be
+[bootstrapped](./bootstrapping-guidelines.md). This process allows the packages from the
+previous source build release to be used to build the next version of the product. This
+provides a means for breaking the product's circular dependencies. For example repos like
+[arcade](https://github.com/dotnet/arcade) can self-reference its previous version to
+produce the next version.
+
+When referencing previous source built packages, it is important to not leak these
+previously built packages into the resulting packages/product. This is considered a
+[poison leak](./leak-detection.md) and is not permitted during a source build as it
+breaks the notion of building the product entirely from source. This hinders the
+ability to service the product.
+
+## Package Versions
+
+Package dependencies that defined using 
+[Arcade's Darc patterns](https://github.com/dotnet/arcade/blob/main/Documentation/Darc.md)
+will get lifted dynamically during a source build if the following conditions are met:
+
+1. The dependency is declared in the Version.Details.xml file.
+
+    **Version.Details.xml**
+
+    ```xml
+    ...
+      <Dependency Name="System.CommandLine" Version="2.0.0-beta4.24068.1">
+        <Uri>https://github.com/dotnet/command-line-api</Uri>
+        <Sha>02fe27cd6a9b001c8feb7938e6ef4b3799745759</Sha>
+      </Dependency>
+    ...
+    ```
+
+1. A corresponding version property is defined in the Versions.props.
+
+    **Versions.props**
+
+    ```xml
+    ...
+      <SystemCommandLineVersion>2.0.0-beta4.24068.1</SystemCommandLineVersion>
+    ...
+    ```
+
+1. A repository reference is defined in the
+ [VMR's project dependency graph](https://github.com/dotnet/dotnet/tree/main/repo-projects).
+This reference does not have to be direct, it can be transitive.
+
+    **{VMR repo project}.proj**
+
+    ```xml
+    ...
+      <RepositoryReference Include="command-line-api" />
+    ...
+    ```
+
+When these conditions are met during a source build, the infrastructure will scan
+the Version.Details.xml file and dynamically create two new Versions.props files
+containing updated version properties for all non-pinned dependencies.  
+
+**PackageVersions.Previous.props:** This will contain version properties with the
+package versions from the [previous release of source build](#previous-source-built-packages).
+If a new package exists that has never been released before, it will not have a 
+version property defined.
+
+```xml
+...
+  <SystemCommandLineVersion>2.0.0-beta3</SystemCommandLineVersion>
+...
+```
+
+**PackageVersions.Current.props:** This will contain version properties with the
+package versions from the [current source build](#current-source-built-packages).
+If a package comes from a repo that has not been built yet, it will not have a version
+property defined.
+
+```xml
+...
+  <SystemCommandLineVersion>2.0.0-beta4</SystemCommandLineVersion>
+...
+```
+
+These two version.props files get imported by the arcade source build infrastructure after
+the repo's Version.props file. Therefore the repo's Versions.props property versions
+get overridden by the source build versions. In the case of the `SystemCommandLineVersion`
+example, the current source build version, 2.0.0-beta4, would win. All msbuild references 
+(e.g. project PackageReferences) to these Versions.props properties pick up the newer 
+versions. This is known as package version lifting since it lifts the originally defined
+package version to the current source built version. This behavior only applies to source
+build in the context of the [VMR](https://github.com/dotnet/dotnet) (see also 
+[Repo Level Source Builds](#repo-level-source-builds)).
+
+### Transitive Version Properties
+
+Transitive version properties in your Versions.props file may not work as intended with
+source build.
+
+**Versions.props**
+
+```xml
+...
+  <MicrosoftBuildFrameworkVersion>17.7.0-preview-23217-02</MicrosoftBuildFrameworkPackageVersion>
+  <MicrosoftBuildVersion>$(MicrosoftBuildFrameworkVersion)</MicrosoftBuildPackageVersion>
+...
+```
+
+**Version.Details.xml**
+
+```xml
+...
+  <Dependency Name="Microsoft.Build.Framework" Version="17.7.0-preview-23217-02">
+    <Uri>https://github.com/dotnet/msbuild</Uri>
+    <Sha>2cbc8b6aef648cf21c6a68a0dab7fe09a614e475</Sha>
+  </Dependency>
+  <!-- No dependency is declared for "Microsoft.Build". -->
+...
+```
+
+In this case source build will override the `MicrosoftBuildFrameworkVersion` to the
+latest version but the `MicrosoftBuildVersion` will remain set to `17.7.0-preview-23217-02`
+because of the property evaluation order. If the desired behavior is for
+`MicrosoftBuildVersion` to be set to the same value as `MicrosoftBuildFrameworkVersion`
+for source build, then you either need to declare the Microsoft.Build dependency
+in the Version.Details.xml file or move the `MicrosoftBuildVersion` assignment outside
+of the Versions.props file.
+
+
+### Repo Level Source Builds
+
+The source build package lifting mechanism is not applicable when building individual
+repos in source build mode because it doesn't have the context of the other product
+repos or previous source build release. In repo source build mode, the versions of the
+packages declared in the Versions.props are used (see also 
+[backlog issue](https://github.com/dotnet/source-build/issues/3562)).