Track --local from gem install for --gemfile

This will allow `gem install --gemfile --local` to work once filtering
of the remote sets is added.

Part of #776
Record the remote access flag per resolver set

This will allow skipping of network access in sets that do access the
network.

Part of #776
Test IndexSet#find_all
Test InstallerSet#consider_local?
Test InstallerSet#consider_remote?
Updating or creating a git cache may be disabled

RubyGems doesn't understand git source URIs so cache creation and cache
updates are disabled regardless of the URI.  Other activities with the
source will work so long as the repository cache dir exists.

Part of #776
Fix unused variable warning
Sets now make network access when remote-enabled

When not remote-enabled network requests return no results.  I'm unsure
what this will do when a gem.deps.rb is not resolvable.

Part of #776
Switch RequestSet#local to #remote to match *Set

This keeps things consistent so I won't get confused switching back and
forth.

Part of #776
Set the remote-enabled status on the resolver set

This allows --local to take effect.  There are still network requests
from BestSet where it probes to see which set to use.

Part of #776
Delay BestSet resolution until needed

Now `gem install --local` does not make network requests.

Fixes #776