Allow secondary git sources to be checked out.

Given:

class FooPackage(Package):
      self.name = "foo"
      self.version = "x.y.z"
      self.sources = ["git://foo.git", "git://secondary.git"]

the repos will be checked into the following locations:

    foo.git => foo-x.y.z
    secondary.git => foo-x.y.z-secondary