Rollup merge of #149400 - Skgland:tracked_mod, r=Amanieu

unstable proc_macro tracked::* rename/restructure

Picking up what should be the uncontroversial part of rust-lang/rust#87173 (closed due to inactivity over two years ago).

Part of rust-lang/rust#99515.

- move `proc_macro::tracked_env::var` to `proc_macro::tracked::env_var`
- move `proc_macro::tracked_path::path` to `proc_macro::tracked::path`
- change the argument of `proc_macro::tracked::path` from `AsRef<str>` to `AsRef<Path>`.

Author: matthiaskrgr