Add back NuGetSourceType=Package metadata in package resolution

[nguerrera]

This was used in a few places to distinguish between refs coming from NuGet from others.

In particular, it caused all nuget refs to be copied to refs/folder on Build with PreserveCompilationContext=true.

Fix #2121

[dsplaisted]

If we control all the consumers of this metadata, I'd prefer we just update them to check if there is a package ID set.

However, this change is also fine if you think it's better based on risk, churn, etc.

[nguerrera]

If [...], I'd prefer we just update them to check if there is a package ID set.

I've discovered that there are other refs with NuGetPackageId but no NuGetSourceType:

https://github.com/dotnet/standard/blob/9a99f024343e6a36d15e7073890c07daa81606fc/netstandard/pkg/targets/netstandard/NETStandard.Library.targets#L25

I don't know yet if it would be a fix or a bug to start treating them as we do the ones that come from assets file. I suspect a fix. cc @ericstj @eerhardt

[eerhardt]

There appears to be other usages of this metadata outside of dotnet/sdk. Doing a quick search:

https://github.com/CommonBuildToolset/CBT.Core/blob/3514c6f2f047a455439dd277437c9596634531b2/src/CBT.Core/ILMerge.targets#L13
https://github.com/dfederm/ReferenceTrimmer/blob/1e4056f8badfda9162cbe2c7f4a5596290bdc3e5/src/Project.cs#L148
https://github.com/Microsoft/WinObjC/blob/d106b1e2009c98723ab02f838aac1d2b264dea88/common/workaround/Microsoft.Nuget.Workaround.targets#L54

[ericstj]

The thing that NETStandard.Library was mimicking is the NuGet restore task that predates the SDK/NuGet's reimplementation. It also sets only NuGetPackageId and NuGetPackageVersion, but not NuGetSourceType: https://github.com/NuGet/NuGet.BuildTasks/blob/8316e6a9ffcec0c0f8ae6a99f1f39bac5417b4e3/src/Microsoft.NuGet.Build.Tasks/ResolveNuGetPackageAssets.cs#L821-L822

[eerhardt]

@ericstj - NuGetSourceType does get set in that code. Just in a different spot:

https://github.com/NuGet/NuGet.BuildTasks/blob/8316e6a9ffcec0c0f8ae6a99f1f39bac5417b4e3/src/Microsoft.NuGet.Build.Tasks/ResolveNuGetPackageAssets.cs#L793

[ericstj]

I see, I stand corrected. Thank you sir.

[nguerrera]

@dotnet-bot Test OSX10.12 Release

(machine was out of disk space)

[nguerrera]

In the links above there is hint of NuGetSourceType being something other than Package. Project seems to be another value. What does this mean? I cannot find anywhere we ever set anything other than Package.

[pranavkm]

@dotnet-bot Test OSX10.12 Release