Make Source#search work optimally again.

[alloy]

This should be part of the removal of source providers other than the file-system one, but I couldn’t find the ticket for that discussion, if it existed at all.

Specifically the Source#search method should not load all of the available pods, because this does not scale at all. The idea of using the file-system as a database is that we can know where that spec should be in the repo, if it exists, and verify it exists with only 1 lookup.

i.e. currently it does (pseudo code):

def search(name)
  all_pods_available_in_spec_repo.find { |pod_name| pod_name == name }
end

Whereas it should work like:

def search(name)
  File.exist?(File.join(spec_repo, name))
end

/cc @segiddins @fabiopelosin