Add a way to force dependencies 'build-requirements' to get built

It seems like testtools has a dependency on a newer version of
unittest2; but we don't seem to have a way to determine this build
time requirement from the extracted egg-info data; so we have to
manually specify it (to be later fixed in a better way if/when that
becomes possible)

Change-Id: Icc294122f937ae98d82b6ef283febcb00fb1e868
Merge "Add a way to force dependencies 'build-requirements' to get built"