Revert "Merge pull request #386 ... gpb-recompilation...detection"

As per discussions with Tuncer Ayaz and Luis Rascão, partly in #410,
and partly as an off-github spinoff private discussion, this is a request to
revert the merge of #386, since it changed the gpb compiler to
not use `rebar_base_compiler`.  That change was to add support for
target name prefixes, but it is better to add that support to
rebar_base_compiler.