Use `module_function` for `Homebrew` module.

brew.sh: don't autoupdate if --help passed

fix tests failing when verbose set

Formula-Cookbook: fix /usr/local/Homebrew references.

virtualenv_install_with_resources: select correct python

Make `MacOS.language` less opinionated and add `language` stanza.

`virtualenv_install_with_resources` will now attempt to guess the
desired Python based on the active build options and based on the
dependencies and requirements. When the situation is ambiguous (e.g.,
`depends_on :python3` and `build.with? "python"` is true) raise
`FormulaAmbiguousPythonError` unless `:using => "python"` or
`:using => "python3"` has been passed to resolve the ambiguity.

In most cases, this will allow

```
    virtualenv_create(libexec, "python3")
    virtualenv_install_with_resources
```

to be changed to just

```
    virtualenv_install_with_resources
```

Add official external commands to manpage.

*/diagnostic: tweak messaging.

list: fix exit code for --verbose and --pinned

These are the ones that are auto-installed without tapping and it makes
sense to make them more prominent in our documentation as we've
offloaded various functionality over the years to each of them.

Update some messaging based on recent user confusion and the
`/usr/local/Homebrew` and Ruby 2 migrations.

update-test: fix tag updating, skip core tap.

Fix updating the `stable` branch used for tagged updates and ensure
that the core tap isn’t retapped (the slowest part by far of this test).

`brew list --verbose <formulae>` lists only those kegs that
are installed and exits with 0 if all <formulae> installed.
If one or more formulae from <formulae> are not installed, the
command lists only installed versions and exits with 1.

`brew --pinned formula` exits with 1 if formula is not pinned.

Fixes #1172

help: support `#:` help in external commands.

install: remove not updated warning.

Fix `Tty.gray`.

Refactor Tty.