Make APICompat friendlier

[ericstj]

Today I had to do the following to demonstrate API Compat usage.

  <PropertyGroup>
    <RunApiCompat>true</RunApiCompat>
    <RunApiCompatForSrc>true</RunApiCompatForSrc>
    <RunMatchingRefApiCompat>false</RunMatchingRefApiCompat>
  </PropertyGroup>
  
  <ItemGroup>
    <ResolvedMatchingContract Include="..\old\apiCompat.dll" />
    <PackageReference Include="Microsoft.DotNet.ApiCompat" Version="1.0.0-beta.19170.2" PrivateAssets="All" />
  </ItemGroup>

  <Target Name="defineContractOutputPath" BeforeTargets="RunMatchingRefApiCompat">
    <ItemGroup>
      <_ContractDependencyDirectoriesTemp Include="@(ReferencePath -> '%(RootDir)%(Directory)')" />
      <_ContractDependencyDirectories Condition="'%(_ContractDependencyDirectoriesTemp.ReferenceSourceTarget)' == 'ProjectReference'" Include="%(_ContractDependencyDirectories.Identity)" />
      <_ContractDependencyDirectories Condition="'%(_ContractDependencyDirectoriesTemp.ReferenceSourceTarget)' != 'ProjectReference'" Include="%(_ContractDependencyDirectories.Identity)" />
    </ItemGroup>
  </Target>

Some notes.

• By default we should run normal API compat, and not reverse API compat. Properties should only be required to turn it off, or to enable reverse compat.
• ResolvedMatchingContract is a terrible name. We should rename it. I've convinced myself I'm OK with this name: contract is good enough and is more appropriate than "reference assembly" since we use API compat for past versions as well.
• We should also have some options for how to define it. Once NuGet download-only packages are available this might be an interesting use-case. In the meantime we should have an example for how to create a project to do this.
  • Sample underway: API Compat tool in ML.NET machinelearning#3623
• When ContractOutputPath isn't defined we should reuse ReferencePath directories as contract dependencies. This should be suppressible with a property. I don't think this is a good idea as it could lead to missed issues in cases where dependencies are also things that might be under test. Instead I've made it easier to pass contract dependencies.

[ericstj]

Taking a look at fixing these as I ran into them again when helping out on dotnet/machinelearning#3623