[Feature]: Address all the package installation rules for package namespaces work in PackageReference and PackageDownload scenarios.

[kartheekp-ms]

NuGet Product(s) Involved

NuGet.exe, Visual Studio Package Management UI, Visual Studio Package Manager Console, MSBuild.exe, dotnet.exe

The Elevator Pitch

The package installation rules in the package namespaces spec has rule: When a package id needs to be downloaded, it is first compared against sources with namespaces. If any match, only those sources are used. If none of the namespaces match, then the sources without namespaces are considered, if any. The second half of the requirement i.e. If none of the namespaces match, then the sources without namespaces are considered is not addressed in NuGet/NuGet.Client#4149 PR. Hence created this follow-up issue to make sure all the package installation rules are handled as per the spec.

Additional Context and Details

No response

[nkolev92]

This one is irrelevant right now, as the default is FullySpecified.

Feel free to reopen if otherwise @kartheekp-ms