This requires updating to Rubocop 0.49.0 which will require some fixes
to rules, in Homebrew/brew and Homebrew/homebrew-core but opening this
for now so I remember.

Improve the messaging around `brew install` when there's a possible user
action such as an `upgrade` or `link` and don't tell people to
`install --force` when it's unnecessary.

While I did this, tweak the output and function usage in a couple of
related places.

Some example output before this change:
```
Warning: openssl is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version
Warning: mysql@5.6 is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version
Warning: analog-6.0_1 already installed
Warning: bash-completion@2-2.5 already installed, it's just not linked.
```

Some example output after this change:
```
Error: openssl 1.0.2k is already installed
To upgrade to 1.0.2l, run `brew upgrade openssl`
Warning: mysql@5.6 5.6.36_1 is already installed
Warning: analog 6.0_1 is already installed
Warning: bash-completion@2 2.5 is already installed, it's just not linked.
You can use `brew link bash-completion@2` to link this version.
```

This works around older versions of `which`, which return a
string on failure.

This reverts commit 3e4547f5, reversing
changes made to 6edf9382.

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.

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

This avoids an error when RuboCop installed installed yet.

CC @GauthamGoli FYI.

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.

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

On Linux this defaults to Linuxbrew but in some cases (i.e. a Linux
machine performing uploads for Homebrew) we want to allow this to be
overridden back to the defaults.

Relies on a change incoming to `brew test-bot` to set this there.

Use GitHub's code search API to search using the filename based on the
search query. This means we only need a single HTTP call and no more
multithreading madness. This also means we're able to search everything
in the Homebrew and Caskroom organisation by default without having to
maintain a list of things to search (and not) in here.

Deprecate more methods. Internal APIs have been verified to be unused
elsewhere and removed. External APIs have had deprecation methods added.
Existing deprecations have been either upgraded to produce warnings or
no longer deprecated and the reasoning documented.

These formulae are detected as missing but exist in an unreadable form.

Fixes #2485

This is useful for seeing when formulae are deleted if they are going to
leave behind any formulae that depend on them.

As a result, if there are any formulae returned return a non-zero/failed
exit status.

These were those migrated from other taps but if they are installed
already: they aren't new to that user.

For example if this is for a really old keg, keg where a user has
manually removed stuff or used `brew diy`.

Commit 4cae6a72 introduced the message,
but it printed the wrong path, e.g. for `brew link sqlite`:

```
If you need to have this software first in your PATH instead consider running:
  echo 'export PATH="/usr/local/opt/sqlite:$PATH"' >> ~/.zshrc/bin
```

where `/bin` is appended at the end, but should be inserted before
`:$PATH`: `echo 'export PATH="/usr/local/opt/sqlite/bin:$PATH"' >> ~/.zshrc`.
This patch fixes that and updates a test to verify it.

This avoids needing to use `force: true` and still let's Homebrew do
what we want with our own taps.