Make PathForArbitraryOutput return an OutputPath

The only place basePath is used as a Path is when being returned from
PathForArbitraryOutput.  Using it as a Path requires implementing the
RelativeToTop() method on it, which then allowed that method to be
inherited by SourcePath, breaking the contract on RelativeToTop that
specifies that the returned Path is of the same concrete type as the
input Path.

Make PathForArbitraryOutput return an OutputPath, and update OutputPath
to support base paths that are not out/soong.  This also fixes
RelativeToTop, which was previously not working for PathForArbitraryOutput
paths.

Test: all soong tests pass
Flag: EXEMPT refactor
Change-Id: Ie8d8e2290961f35280e97137d2bd641c4d57ab87