Collect guix profiles in single directory.

[Leo Prikler]

Hello Guix,

currently, our configuration is (more or less true to XDG) scattered
around various places.  Even if one does not make use of more advanced
features, one will at least have two profiles:
- ~/.config/guix/current
- ~/.guix-profile
If one does have more configuration data, the channels.scm must be put
under ~/$XDG_CONFIG_HOME/guix/.  Note, that this need not be
~/.config/guix, but most likely is.  If one wishes to use more than one
profile, so as to keep the build times for each shorter, one has to
write custom code and can not rely on /etc/profile.

This is a proposal to change the current approach.  Instead of
splitting Guix config in a way that tries to conform to XDG, but does
not really, I propose to use a single directory ~/.guix for all
configuration.
This includes:
- ~/.guix/channels.scm
- ~/.guix/channels (instead of ~/.guix/current, to match the name
channels.scm)
- ~/.guix/profile
- user-generated profiles

/etc/profile would be rewritten to assume that all subdirectories of
~/.guix are profiles, but loading ~/.guix/channels at last, as is the
current behaviour.  In addition, the crucial variables, that are
exported manually for #20255 and #24445 would also have to be taken
into account here.

I think, that these changes would make working with multiple profiles a
lot easier, as all profiles in that directory would then be considered
equal (with the exception of channels).  Additionally, a command could
be provided to operate on all profiles at once, to allow cumultative
upgrades, roll-backs, etc. although that might as well be done through
user scripts (that they could also put into ~/.guix).

WDYT?

Regards,
Leo