Give GemDepedencyAPI#load a useful return value

This will allow the lockfile to know the gems the gem.deps.rb required.
Fix GemDependencyAPI#gem_requires test names
Record dependencies from gem.deps.rb

This will allow filling in the DEPENDENCIES in a lockfile correctly
Refactor Lockfile#add_DEPENDENCIES

This will enable use of GemDependencyAPI#dependencies in a more-friendly
manner
Set requirement for git, etc. gems in gem.deps.rb

For git, github and path gems from gem.deps.rb the lockfile will contain
a '!' requirement.  Recording this when reading the gem.deps.rb makes
writing it back out properly easier.
Match bundler's DEPENDENCIES in gem.deps.rb.lock

Previously the lockfile did not contain the correct entries for
dependencies.  Only the gems listed in the gem.deps.rb should be
included.  Instead of using the gems from gem.deps.rb
RequestSet#lockfile tried to figure this out by itself.

Now the dependencies from gem.deps.rb are used directly.
Test Lockfile#add_PLATFORMS
Do not exclude ruby platform in lockfile

This confuses bundler and causes it to no longer look for ruby gems.
Ignore bundler in GEM lockfile section

Bundler ignores itself and fails to use a rubygems-generated lockfile in
some situations when bundler shows up in the GEMS list.

Now we also ignore bundler when generating the lockfile.