Do not copy subpackages if they are not imported
Blocked by #22 and #27
For example fetching golang.org/x/oauth2 should not copy golang.org/x/oauth2/google, and so google.golang.org/cloud/compute/metadata should not be fetched.
However this is a breaking change for restore as subpackages people are relying on might not be explicitly set in the manifest. So needs a "NoSubpackages" default on option. Sigh restore.
The transition should be handled by the purge command (#16)
@davecheney I am about to change this behavior, so I thought I would ask you what was the original reason for the design (vendoring all the subpackages). Thanks :)