Prevent transitive import of MSBuild tasks and Source Generator

[Copilot]

MSBuild tasks and Source Generators were being transitively imported when a project referenced a library using LuYao.ResourcePacker.MSBuild. Projects depending on such libraries should only get the runtime library, not build-time functionality.

Changes

Package structure

• Removed buildTransitive/ folder to prevent transitive import of MSBuild tasks/analyzers
• Set PrivateAssets="none" on runtime library reference to allow transitive dependency flow
• Kept PrivateAssets="all" on Source Generator to block transitive flow
• Added MSBuild.dll to tasks folder in package

Build property initialization

• Removed ResourcePackerOutputFileName from .props (was evaluated before AssemblyName initialized, causing .dat filename)
• Moved to .targets using MSBuild::ValueOrDefault('$(AssemblyName)', '$(MSBuildProjectName)') for proper fallback

Behavior

LibA (references LuYao.ResourcePacker.MSBuild)
  ✓ MSBuild tasks execute, generates LibA.dat
  
LibB (references LibA)  
  ✗ No MSBuild tasks (non-transitive)
  ✓ Gets LuYao.ResourcePacker runtime library (transitive)
  ✓ Gets LibA.dat in output (via CopyToOutputDirectory)
  
App2 (references LibB)
  ✗ No MSBuild tasks
  ✓ Gets runtime library  
  ✓ Gets LibA.dat in output

Test scenario

• Added test-scenario/ demonstrating LibA → LibB → App2 chain with expected behavior

Original prompt

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This section details on the original issue you should resolve

<issue_title>msbuild和sg项目不应该传递依赖。</issue_title>
<issue_description>如果类库A引用了 MSBuild 项目，那么 MSBuild 应该仅限于 A 项目。依赖 A 的项目不应该默认引用 MSBuild 和 SG 项目，需要单独引用。</issue_description>

<agent_instructions>请创建两个类库一个控制台项目，形成链式依赖。并在依赖的顶层引用 LuYao.ResourcePacker.MSBuild 项目（通过 NuGet 的形式）。
假设分别是 LibA、LibB 和 App2 。其中 LibA 通过 NuGet 引用了 LuYao.ResourcePacker.MSBuild 并添加了需要生成 dat 的文件。那么，LibB、App2 中如果需要同样的功能，就需要单独再次引用 LuYao.ResourcePacker.MSBuild ，而不是直接将依赖传递。

这个能实现吗？是否会导致 LibA 生成的 dat 文件无法出现在 App2 的生成目录中？</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes msbuild和sg项目不应该传递依赖。 #27

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