Support optional /src/ in jumping between test/implementation #4773

Sometimes implementations live in src but the tests live in the same path without src:

The "Go to Test/Implementation" command currently assumes a perfect match. We should handle when src exists/doesn't as a fallback.

We should probably also support tests alongside the implementation (like lib/src/foo.dart <-> lib/src/foo_test.dart) since we have some other support for tests outside of test for users that like to keep them together.