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.