Unvendor log15 library

[gibmat]

Issue description

While continuing to work on the LXD package for Debian, I noticed that there is an essentially unmodified copy of the log15 library vendored in the shared/ directory. I did find the (still open) bug about the API breakage (inconshreveable/log15#139) and commit 30c8594 that was applied to fix the build at the time.

Today I was able to successfully build LXD 4.21 with v2.15 of log15 after fixing up the imports and re-applying the commit referenced above. It seems that the log15 v2 API has remained stable for a few years now, or at least the parts of it that LXD uses. Would you consider unvendoring that code and depending on the upstream library again?

Debian packaging policy strongly frowns on vendored copies of code, especially when they're simple copies of upstream code. At the moment I've got a large (but simple) patch to remove the copy of log15 from the LXD source, but I'd prefer not to have to maintain that if we can make this change in LXD directly. :)