(Summary: you want to get the go code for import foo/bar/baz/quux. The ArchiveFetcher needs to figure out where the split is between an "import-prefix" (which is where you get to the tarball), and the "remote package" (which is the directory within that tarball). Today it does this by depending on the regex in the matcher matching exactly through the import-prefix. This is not useful if you want to send everything to artifactory, so you need another way to specify the import-prefix'es that you know about. So this adds the ability to specify them in another file (to prevent pants.ini from growing without bound and each time you want to use a new go library). (Outside of pants, the go command would use its protocol to read the meta tag for go-import from the path listed)