Make gem install --local not attempt remote access

gem install --local has been attempting remote access since
ccde1e0e2e4dba6a7e0951aab37b70b9a93429db was committed back
in February.  This is because it prefetches even in --local
mode.  Skip prefetching if --local is used, so that no remote
access happens in that case.
Add a test for InstallerSet#prefetch

This is overly simplistic, just testing it prefetct is called
on the remote_set, but it at least tests that it is called in
remote mode and not called in local mode.
Merge pull request #1084 from jeremyevans/local

Make gem install --local not attempt remote access