Add more methods for resolving definitions from AST to their corresponding HIR types
In order to be able to add these methods with consistent naming I had to also rename two existing methods that would otherwise be conflicting/confusing:
`Semantics::to_module_def(&self, file: FileId) -> Option<Module>` (before)
`Semantics::file_to_module_def(&self, file: FileId) -> Option<Module>` (after)
`Semantics::to_module_defs(&self, file: FileId) -> impl Iterator<Item = Module>` (before)
`Semantics::file_to_module_defs(&self, file: FileId) -> impl Iterator<Item = Module>` (after)
(the PR is motivated by an outside use of the `ra_ap_hir` crate that would benefit from being able to walk a `hir::Function`'s AST, resolving its exprs/stmts/items to their HIR equivalents)
d8feb908be