Allow correct activation of path: gems

Previously RubyGems assumed that a path: gem lived in the same structure
as a regular gem.  This would mean use of a gem.deps.rb would not work
correctly as the files in the gem would not be accessible through
$LOAD_PATH.

Now the root of the gem's files can be set on a specification to allow
correct operation.
Set full_gem_path on a git: gem's specification

This allows the gem to be activated properly.
Fix fetching of remote specifications via APISet

The default Resolver::Specification#installable_platform? fetches the
remote spec, which is usually cached.  For APISet it is not.

Now we match directly with the platform from the API instead of looking
for the remote specification.