create placeholder Build.PL in filemunging phase, to allow other plugins...

... to see that this file exists

e.g. my templated CONTRIBUTING file wants to see if Build.PL exists in the file list, so it can add some content that mentions this file. (currently the only way to do this is to examine $dist->plugins for /ModuleBuild/)

similar patches exist for [MakeMaker] and [ModuleBuild] at https://github.com/rjbs/Dist-Zilla/pull/229