GH-129835: Yield path with trailing slash from ReadablePath.glob('') (#129836)

In the private pathlib ABCs, make `ReadablePath.glob('')` yield a path with
a trailing slash (if it yields anything at all). As a result, `glob()`
works similarly to `joinpath()` when given a non-magic pattern.

In the globbing implementation, we preemptively add trailing slashes to
intermediate paths if there are pattern parts remaining; this removes the
need to check for existing trailing slashes (in the removed `add_slash()`
method) at subsequent steps.

Lib/pathlib/_abc.py

@@ -460,7 +460,7 @@ class ReadablePath(JoinablePath):
         recursive = True if recurse_symlinks else _no_recurse_symlinks
         globber = _PathGlobber(self.parser.sep, case_sensitive, case_pedantic, recursive)
         select = globber.selector(parts)
-        return select(self)
+        return select(self.joinpath(''))
 
     def rglob(self, pattern, *, case_sensitive=None, recurse_symlinks=True):
         """Recursively yield all existing files (of any kind, including