Allow multiple identical sources

The source-repositories element puts source in place in the generated image
for later installation.  It is possible for two elements to define the same
source to later install, and as long as the remote location and reference to be
installed is identical, we should not error.

Change-Id: Ic7dfb1961486c59d2005b5c1e7587267ea42e999
Merge "Allow multiple identical sources"