ENH: RFC bundle fixup script in python

See commit message for more detailed information. The goal here is to make the packaging system easier to use and debug. Using python allows for shorter code, and makes the fixup step much faster: the total packaging time is reduced to ~3 min as compared to ~1 hr (make package in an up-to-date build tree). The script can also be debugged with pdb. Hopefully this makes other build system improvements easier to test and iterate, because the turn-around to test changes will be shorter.