If people have `HOMEBREW_RUBY_PATH` set then things explode in a rather
confusing fashion. Instead, run `bundle` for them with the arguments
that they'd want.

Also, move `macho` requires into the module itself; it's a pain having
to do everything for Bundler before requiring `pathname` which is a core
Ruby class.

Vendor all Homebrew's gems.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.

Remove unused `e` from `rescue Interrupt`.

Remove unused command_args

Extra command_args in gain_permissions_remove caused silent failure and path never gets deleted.

style: fix audit --online

This passed a symbol to `:except_cops` which caused a .

style: don't require rubocop until installed.

This avoids an error when RuboCop installed installed yet.

CC @GauthamGoli FYI.

Remove `existing` from PATH when adding `Gem.bindir`.

audit: Allow skipping/selective running of cops and cops refactor

Install a bottle from an URL

caveats: tweak keg_only style

Install etc/var files on postinstall.

utils: create GEM_HOME when installing Gems.

Also, don't delete them after that. This means that `brew postinstall`
becomes a way to easily reinstall configuration files for any formula
without needing any changes to any bottles or requiring a reinstall.

It may not exist before Gem installation which means that the resulting
installed gem will not be found in the PATH.

Refactor PATH generation.

Also refactor audit cops into two "departments"
 - FormulaAudit
 - FormulaAuditStrict

caveats: tweak keg_only wording

Current:
```
This formula is keg-only, which means it was not symlinked into /usr/local.

Qt has CMake issues when linked
```

After:
```
This formula is keg-only, which means it was not symlinked into /usr/local,
because Qt has CMake issues when linked.
```

Revert "Fix `IO#select` blocking."