SDK Manager: fix suggestions of new platforms.

SDK Manager: fix suggestions of new platforms.

I accidentally broke the way the SDK Manager was suggesting
new platforms when I "optimized" the way it was fetching new
sources when resolving dependencies. The fix is that even if
we don't need to refresh or fetch a source, we still need to
report the packages we know it contains.

Also made the Archive and ArchiveInfo implement Comparable
and defer their comparison to the one of Package. This way
we can sort the archives in the install window.

Change-Id: Ic3b39e49e8143541b19b00de09468c1b3f01b0d7