Auto merge of #37602 - jseyfried:directory_ownership, r=nikomatsakis

parser: simplify directory ownership semantics

This PR simplifies the semantics of "directory ownership". After this PR,
 - a non-inline module without a `#[path]` attribute (e.g. `mod foo;`) is allowed iff its parent module/block (whichever is nearer) is a directory owner,
 - an non-inline module is a directory owner iff its corresponding file is named `mod.rs` (c.f. [comment](https://github.com/rust-lang/rust/issues/32401#issuecomment-201021902)),
 - a block is never a directory owner (c.f. #31534), and
 - an inline module is a directory owner iff either
   - its parent module/block is a directory owner (again, c.f. #31534), or
   - it has a `#[path]` attribute (c.f. #36789).

These semantics differ from today's in three orthogonal ways:
 - `#[path = "foo.rs"] mod foo;` is no longer a directory owner. This is a [breaking-change].
 - #36789 is generalized to apply to modules that are not directory owners in addition to blocks.
 - A macro-expanded non-inline module is only allowed where an ordinary non-inline module would be allowed. Today, we incorrectly allow macro-expanded non-inline modules in modules that are not directory owners (but not in blocks). This is a [breaking-change].

Fixes #32401.
r? @nikomatsakis