do not search for the file object if we already know it
put Build.PL back in the file list so other plugins can see it

e.g. plugins that want to insert some content; also prevents
[VerifyPhases] from complaining about Build.PL added late
Dist-Zilla-Plugin-ModuleBuildTiny-Fallback-0.010

          - keep Build.PL in the file list between phases to allow other
            plugins to add to its content