Fix for absolute and foreign relative paths in Compile.Include for "already exists" tests against csproj

Fixed up relative path building to use built-in algorithm which is more robust. This resolves an issue where relative paths pointing in a peer tree to the project.csproj would cause the server to rewrite csproj's with absolute paths that were missing the volume char (at least on Windows).

Also added direct support for absolute paths stored in the csproj's in case there are any of those.
Does the old version of this test still work? Could you create a new test please?