Report proper errors on item name conflict and write more tests

[froystig]

Conflicting item names at the same path level currently get caught by assertions in the compiler. Report friendly errors instead. This could also use some more thorough testing, so write a few tests (both failing and not) beyond the current run-pass/item-name-overload.rs and compile-fail/item-name-overload.rs.

[graydon]

WONTFIX (not required for bootstrapping)