Make repo take precedence over sig/ in gem package

[pocke]

This PR changes loading priority between gem_repo and sig/ in gem package. Repo will take precedence over sig/.

Currently sig/ in gem package takes precedence over gem_repo. It means if there're RBSs in both of gem_repo and sig/, sig/ is used.
It is not a useful behavior. We can modify gem_repo's content easily, especially if we use rbs collection. But we need to fork the gem to modify sig/ directory in the gem package.
So, overriding RBS in sig/ with gem_repo is useful, but the opposite isn't.

I propose this PR to enable the overriding.

---

By the way, I'm wondering if EnvironmentLoader can receive the sources of RBS, which are gem_repo, sig/, or stdlibs.
I guess this PR is ok in most cases, but we need to specify the source to EnvLoader to make 100% sure that the loader loads RBS from expected places. I'll consider this problem 🤔

[soutaro]

I agree that adding an option to EnvironmentLoader is better way. Something like this?:

class EnvironmentLoader
  def add: (path: Pathname) -> void
         | (library: String, version: String?, ?from: Repository | :gem | nil) -> void
end

I'm still not very sure if making repository priority over gem by default is a good idea or not...

[pocke]

I'll close this issue. I and @soutaro discussed this PR and we agree with adding the from option.