Rollup merge of rust-lang#102869 - azdavis:master, r=joshtriplett

Add basename and dirname aliases

Users might be used to the POSIX names of these functions. In fact, here's a [blog post][1] about this very thing.

[1]: https://boinkor.net/2019/07/basename-and-dirname-in-rust/

Author: matthiaskrgr