Cache repository-sources data.

Cloning large repositories over the internet can take considerable
time.  Caching them locally makes repeated image builds significantly
faster, so lets do that.

When users override the element source they will often be using a
local repository, so in those cases I don't cache - but we could
easily change our minds on that in the future.

Change-Id: I9822362cf722b904d9806dbbb4bb07cfe2b33437