fix(gazelle): Correctly resolve deps that have top-level module overlap with a gazelle_python.yaml dep module

[alex-torok]

In #2048, FindThirdPartyDependency was updated to walk up the module import path to try to find a match in gazelle_python.yaml. This is unnecessary, as the main resolve loop will accomplish the same task.

Additionally, the change broke the ability to configure a more specific resolve override or resolve more specific indexed libraries.

For a real-world example of where this is a problem, pytype has a third_party module at its top-level. In a repo that also has a third_party directory, we can no longer resolve our indexed libraries in third_party. When the resolve loop tried to resolve third_party.foo.bar, FindThirdPartyDependency will immediately match third_party and not give the resolve loop a chance to look in the rule index for third_party.foo.bar.

The same issue appears for providing overrides that are more specific (see the updated testcase).

This PR reverts the change to FindThirdPartyDependency and updates the testcases to ensure that we can still resolve specific indexed packages, explicit resolve overrides, and third party modules even when there is an overlap in the top-level module name.

[alex-torok]

FYI @aignas since you reviewed #2048.

[aignas]

Thanks