chore(perf): Make all gems load faster

[gmalette]

Gem.find_latest_files's performance scales with the amount of files contained across all gems of a project. This can quickly get out of hand, especially on projects with many gems, or large gems.

Gem.loaded_spec is already populated by bundler, and we can iterate over only the name of the gems to detect those matching the google-cloud-* pattern. We can then guess the name of the file we expect to be present.

On my machine, this makes auto_load_files go from ~100ms to <1ms, and requiring bigtable goes from ~220ms to ~110ms.

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea.
• Follow the instructions in CONTRIBUTING. Most importantly, ensure the tests and linter pass by running bundle exec rake ci in the gem subdirectory.
• Update code documentation if necessary.

[gmalette]

I don't understand what the CI failure is