Make `HOMEBREW_PREFIX/Caskroom` the default.

Make `brew upgrade` and `brew outdated` follow alias changes

brew.sh: fix repository auto-update change.

On auto-update `HOMEBREW_LIBRARY` may change location which means that
it won't be found for the actual install command. Look for this having
occurred and then set the new `HOMEBREW_LIBRARY` (and
`HOMEBREW_REPOSITORY`) accordingly.

More Rubocop tweaks

Closes https://github.com/Homebrew/homebrew-core/issues/4973

References #1023.

Can't see how this could ever happen but let's safe-guard it just in
case (as ilovezfs saw it).

update-report: migrate everyone to new repository.

Use new "macOS" naming where appropriate.

update-report: remove cask dir after migration.

keg: only prune var/homebrew/linked.

Relocate HOMEBREW_REPOSITORY when necessary.

This will make brew readall --syntax pass

Previously, this method would:

- Include the current formula
- Include the current target of the installed alias, which by definition
  is not "old"

This commit fixes both of these issues.
Only formulae that are the current target of the alias they were installed
with are now considered to have old installed formulae.

`var/homebrew` is too broad and ends up deleting and recreating the
locks directory repeatedly.

This will hang around otherwise due to e.g. running `bundle install` in
here once.

update-report: cleanup git harder in migration.

Before stashing also try to cancel any current merge, rebase and
reset. This will prevent any data loss but also make the stash more
likely to succeed.