prevent module being enabled if in existing BUILD file

If a module is enabled via the module-level allow list,
bp2buildModuleAlwaysConvertList, then we should error if that same
module is hidden by an existing BUILD file.

Bug: 220875679
Test: add a module in a directory covered by an existing BUILD file
Change-Id: I5426bd9edac593690c0989b68a421ac1c3216010