The specific requirement for ABI checking is:

Download a file containing "abi check failure suppression information" from a specific relative location in dist-git

But I don't see why this should be so specific - I imagine that we'll be asked to support downloading other files from dist-git before too long. So I propose that we add a distgit-download directive that looks like

distgit_download:
  package: $item
  path: somedir/somefile
  localpath: someotherdir/someotherfile

Implement the new directive with the following requirements:
* the package input should be smart enough to figure out the package name from a nvr.
* package and path will be required
* localpath is optional (default is to use the same path/filename as the source)
* any missing local directories will need to be created, relative to the formula's containing directory