[build] Import "parent directory.override.targets" (dotnet#135)

Context: ff73f92
Context: xamarin/XamarinVS#12550

The XamarinVS team would like to *not* use `GitInfo` to set the
`$(Version)` MSBuild property (commit ff73f92), and would like to
instead version *all* of their dependencies with the same version.

Support this behavior by by updating `Directory.Build.targets` to
import "parent directory.override.targets", a'la
[xamarin/java.interop@8898bc][0] and [xamarin/java.interop@9a878f][1].
For example, if `xamarin-android-tools` is checked out into:

	external/xamarin-android-tools

then we'll import `external/xamarin-android-tools.override.targets`
from `Directory.Build.targets`.

The `<Import/>` of the parent-directory `.targets` file *must* be
*last* within `Directory.Build.targets`, so that it can override any
targets in our default `Directory.Build.targets`, e.g. the
`SetVersion` target.

[0]: dotnet/java-interop@8898bc1
[1]: dotnet/java-interop@9a878f2

Directory.Build.targets

@@ -13,4 +13,9 @@
       <InformationalVersion>$(Version); git-rev-head:$(GitCommit); git-branch:$(GitBranch)</InformationalVersion>
     </PropertyGroup>
   </Target>
+
+  <Import
+      Project="$([System.IO.Path]::GetDirectoryName($(MSBuildThisFileDirectory))).override.targets"
+      Condition=" Exists('$([System.IO.Path]::GetDirectoryName($(MSBuildThisFileDirectory))).override.targets') "
+  />
 </Project>