create Build.PL during filegathering stage, so other plugins can see it

Other plugins, e.g. [InstallGuide], look at which of Makefile.PL and Build.PL exist, in order to generate its content. Moving the file creation to the GenerateFile stage allows that plugin to correctly create its content in the filemunging stage, rather than running as an InstallTool.

(a similar PR is being submitted for [MakeMaker] and [ModuleBuild].)