Invoke GetBuildVersion MSBuild task only once per project (or per repo)

[AArnott]

A multi-targeting project will execute the GetBuildVersion target for every target framework, yet the result will never vary across those target frameworks. Since the GetBuildVersion task is costly to run, we can significantly reduce wasted build time in the multi-targeting scenario by invoking the task within a special project that never varies by TargetFramework and then communicating the result back to the calling project.
This multi-targeting optimization is always on with no effort on the consumer's part.

In fact, because MSBuild will cache the results based on project file and global properties, we can even allow the repo to control the global properties such that they get exactly one GetBuildVersion invocation for the entire repo's build.

In the most trivial case, a repo can define a Directory.Build.props file that defines the MSBuild property GitVersionBaseDirectory as $(MSBuildThisFileDirectory), at which point all projects under that directory will reuse the result of the version calculation from just that one directory. Like this:

<Project>
  <PropertyGroup>
    <GitVersionBaseDirectory>$(MSBuildThisFileDirectory)</GitVersionBaseDirectory>
  </PropertyGroup>
</Project>