fix: better return type inference when reading the body of relation methods

[canvural]

Related to #531

This PR provides better support for return types of relations methods when reading their body.

cc @ondrejmirtes

[ondrejmirtes]

Found one thing :) Otherwise 👍 (if it works form an integration PoV)

[canvural]

Is there a point of caching the result of analyzing the method body, in the extension? Or the performance gain would be negligible?

[ondrejmirtes]

@canvural I don't cache it in phpstan-doctrine and it's fine. You can do a simple in-memory cache by an array in private property if you're concerned. That way, the AST will not be traversed multiple times looking for the same method during the same PHPStan run.

[canvural]

@ondrejmirtes Yeah, that was what I had in mind. Simple array in the extension with filename and method name as the key.

But first, we can go without it, and if we see some issues we can add it later.

Thank you for the review!