Support to call collection coomand in chid dir

[pocke]

This PR takes over #1025.

How to find rbs_collection.yaml and Gemfile.lock

It searches these files with the following procedure.

rbs_collection.yaml

It searches this file upwards. Search ./rbs_collection.yaml, then ../rbs_collection.yaml, then ../../, ...

Gemfile.lock

It uses Bundler.default_lockfile, which is the same as bundle exec's Gemfile.lock. If rbs collection is not executed with bundler, this method searches Gemfile.lock with the same procedure as rbs_collection.yaml.

Mismatch between rbs_collection.yaml and Gemfile.lock

As #1025 (comment), rbs_collection.yaml refers only to one Gemfile.lock. If it refers to a different Gemfile.lock, rbs collection install generates inconsistent rbs_collection.lock.yaml.

rbs_collection.lock.yaml records gemfile_lock_path to avoid this problem. If Bundler.default_lockfile points to a different path from gemfile_lock_path, it raises an error.

This change extends the syntax of rbs_collection.lock.yaml, but it is not a breaking change because the old format is still supported. If rbs_collection.lock.yaml has no gemfile_lock_path, it doesn't check anything and record current Bundler.default_lockfile as gemfile_lock_path.