borg2: use platformdirs python package?

[ThomasWaldmann]

@RayyanAnsari suggested to use the platformdirs python lib ( https://github.com/platformdirs/platformdirs ) and I had a closer look now:

• the code quality seems decent, from an experienced developer
• the amount of code is not much, but also not trivial. it might be also interesting for us that it supports android also (but not sure it is relevant for termux users).
• some dists have it already packaged, some do not (looked at debian,ubuntu,fedora), so there might be some work for packagers.
• there is 1 open ticket in their tracker with some discussion whether they use some (macOS) directory correctly or not. macOS user_config_dir goes against OS guidelines tox-dev/platformdirs#98
• so, i guess we could add it as a python dependency for borg if we can get rid of some of the corresponding code we have.
• we would need to be careful, so we do not break "borg transfer" when it reads borg1 archives/repos (config/cache access the borg1 way).
• for borg2 config/cache, we could be incompatible as that would be all-new anyway.
• it seems to not yet autocreate not-yet existing directories, but guess the maintainer there is open for a PR with that. Auto-create the dirs tox-dev/platformdirs#120

[RonnyPfannschmidt]

I highly recommend the library

[ThomasWaldmann]

XDG_*_HOME

Not having XDG_*_HOME with platformdirs on macOS caused a bit of a pain (and might also cause upgrade issues).

tox-dev/platformdirs#4

get_base_dir

Guess we need to change a bit how this works in borg:

• old: use BORG_BASE_DIR env. If not set, make something up from HOME/USER/~.
• old: the caller does use BORG_DIR, falls back to something in BORG_BASE_DIR, falls back to XDG_HOME, falls back to HOME/USER/~.
• new: just use BORG_BASE_DIR env, if set, leave everthing else to caller.
• new: the caller does use BORG_*_DIR, falls back to something in BORG_BASE_DIR, falls back to platformdirs.get_* (which deals with XDG stuff on some platforms).