Reduce Proton installations across multiple steam libraries

Feature Request

I confirm:

• [x ] that I haven't found another request for this feature.
• that I have checked whether there are updates for my system available that
  contain this feature already.

Description

Whilst performing some investigation for (https://github.com/ValveSoftware/Proton/issues/2241) I noticed that the same proton version was installed per steam library, rather than only once. Ideally, I only want one copy of proton 3.16-6 (or whatever other proton version I'm using) on my system.

As a manual fix I deleted most the Proton installations but, kept the copies residing under ~/.local/share/Steam/steamapps/common/ as that's my SSD and I have a need for speed. Then, I made symbolic links for Proton 3.7-8 / 3.16-4 / 3.16-6 to my other steam libraries across my HHD's and SSHD.

Justification [optional]

I don't not have my system WINE installed on each of my hard drives, it's installed once on my SSD. Why should proton be installed multiple times?

Risks [optional]

Reduced disk space usage.

References [optional]

The issue names no source files, tests, or entry points. Begin by tracing how Proton versions are installed into the Steam library paths described in the report, with issue 2241 as related context; done means one Proton version is stored once and usable from multiple libraries without manual symlinks.