This implements:

* Supporting user-defined environment variables adhering to the [XDG
Specification](https://specifications.freedesktop.org/basedir-spec/basedir-spec-0.6.html)
to override yarn's defaults
* Supporting equivalent environment variables on Windows such as
%LOCALAPPDATA%
* More conventional defaults for these locations according to the
operating system.
* Support for the user defined config dir in the `.yarnrc` lookup path
* Storing global modules in a data-oriented location rather than a
config location (seeing this is actually what motivated this PR)

Concerns:

* Existing Windows config locations will break. This probably need to be
addressed with a migration path and/or a breaking change
* A few notes included in comments (will highlight these with inline GH
comments)
* Unclear test status as master builds appears to fail on my laptop as
well.

Really interested in your feedback. I know [this has been attempted
before](https://github.com/yarnpkg/yarn/pull/3674/files) -- cc @kelseasy
-- and I'd really like to get this in!

Will Binns-Smith added 2 commits

wbinnssmith pushed a commit to wbinnssmith/yarn that referenced this pull request

Following from yarnpkg#5336, this makes the typical directories for XDG and
Windows conventions the default.

This RFC is really carried over from yarnpkg#5336, where we continued to use
the existing locations inside ~/.config and punted on migrating users to
these new locations.

A suggestion from the prior PR include moving the contents of
the existing directories (should they exist), leaving a symlink in
place.

I really like this approach, however there will be a cost to checking to
see if these directories exist and leaving symlinks behind in a
synchronous way to maintain compatibility with how we expose constants
synchronously from a module.

Please chime in!