Make Rails engine path check more specific

[vinistock]

Motivation

We load Rails engines that are defined inside the current gem's directory and extract symbols to process from that engine's eager auto load paths.

When Rails is installed through a release version, this works perfectly because each gem is placed in a sibling directory

~/.gem/ruby/3.3.3/gems/rails-7.1.4
~/.gem/ruby/3.3.3/gems/actionmailbox-7.1.4
~/.gem/ruby/3.3.3/gems/activestorage-7.1.4
...

However, when Rails is installed via a git source, Bundler nests all of the sub-gems inside the same directory

~/.gem/ruby/3.3.0/bundler/gems/rails-e3ea4c74124f/
~/.gem/ruby/3.3.0/bundler/gems/rails-e3ea4c74124f/activestorage
~/.gem/ruby/3.3.0/bundler/gems/rails-e3ea4c74124f/actionmailbox
...

The result is that we get symbols that belong to the sub-gems (activestorage, actionmailbox) placed inside the Rails RBI, which is incorrect. Note that this problem only happens if another gem eagerly requires one of the engines.

Implementation

Currently, we are checking if the engine's root is anywhere inside the gem's full path.

I'm proposing that we changed to be more specific and only check if the engine root is exactly equal to the gem path.

I don't know the original logic was intentional. If we do need to check sub-directories, then I'm not sure how we would go about fixing this. Maybe we can verify if the gem was installed through a git source and only do the more specific matching in that case?

I did verify this change on Core and everything looks fine.

Tests

Added a test that reproduces the behaviour.

[paracycle]

Great find, thank you!

I wonder if there are any gems that set up and configure their engines to use a subdirectory of the gem path as the root path, but I doubt that's something we have to address now. If someone complains, we can address it then.