Make dest_base/repo@subrepoName/src/contrib/repo.R THE canonical
source for manifest, previous build results, etc.

lazyRepo (and thus Install) function that can handle all source types.
	 Use PkgSource class instead of hardcoding for Github

Results should be in repo@results, no longer in repo@manifest (which
is now a PkgManifest object that doesn't have any prev or current
build info in it)
