Revisit require lookup paths

[straight-shoota]

The documentation about requiring files and the actual implementation in Crystal::CrystalPath have some discrepancies.
This is not just a documentation error and instead some of the documented behaviour should indeed be implemented.

This topic was previously discussed in #10820, but it apparently only covered a part of it. At least since that discussion we have explicit specs about the compiler behaviour. That should make it easy to follow what's happening in the compiler. Actually, in the PR that
introduced the specs (#10876), I explicitly mentioned that we should probably make some changes. But there never was a follow-up.
So here it is now after it was brought up again in crystal-lang/crystal-book#677.