Stabilize std::path

This commit stabilizes essentially all of the new `std::path` API. The
API itself is changed in a couple of ways (which brings it in closer
alignment with the RFC):

* `.` components are now normalized away, unless they appear at the
  start of a path. This in turn effects the semantics of e.g. asking for
  the file name of `foo/` or `foo/.`, both of which yield `Some("foo")`
  now. This semantics is what the original RFC specified, and is also
  desirable given early experience rolling out the new API.

* The `parent` function now succeeds if, and only if, the path has at
  least one non-root/prefix component. This change affects `pop` as
  well.

* The `Prefix` component now involves a separate `PrefixComponent`
  struct, to better allow for keeping both parsed and unparsed prefix data.

In addition, the `old_path` module is now deprecated.

Closes #23264

[breaking-change]

src/libsyntax/parse/parser.rs

@@ -5254,13 +5254,7 @@ impl<'a> Parser<'a> {
                     -> (ast::Item_, Vec<ast::Attribute> ) {
         let mut prefix = PathBuf::new(&self.sess.span_diagnostic.cm
                                            .span_to_filename(self.span));
-        // FIXME(acrichto): right now "a".pop() == "a", but need to confirm with
-        //                  aturon whether this is expected or not.
-        if prefix.parent().is_some() {
-            prefix.pop();
-        } else {
-            prefix = PathBuf::new("");
-        }
+        prefix.pop();
         let mut dir_path = prefix;
         for part in &self.mod_path_stack {
             dir_path.push(&**part);