Cleanup documentation site

GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.

audit: deprecate env :std/:userpaths for strict.

This should apply only for new formulae but we should start gradually
phasing it out for older ones too.

formula: make prefix usually return opt_prefix.

Deprecate Homebrew/versions, Homebrew/devel-only.

General documentation updates.

Don't use Homebrew/versions in a test and remove them both from the
OFFICIAL_TAPS list (i.e. `brew search`).

update-report: handle homebrew/versions imports.

Mostly related to the newer ways we’re handling versions and the
incoming deprecation of devel-only, head-only and versions.

Disallow file separator in version strings.

Imports from homebrew/versions are migrated from that tap and then
renamed immediately when they hit homebrew/core. This did not trigger
our previous rename detection so address these to improve the output and
handle migration correctly.

bottles: --force-bottle feigns or_later.

When reproducing issues with software that hasn’t been bottled yet on
your version of macOS it can sometimes be helpful to use `or_later`
bottle functionality i.e. just use the bottle for the latest version of
macOS available. This maps well to the existing `--force-bottle`
argument so it will now act as if the latest bottle has a `or_later`
ending.

cask depends_on arch: remove everything but 64-bit intel

keg: update installed dependency resolution

Closes #1525. (See there for a motivation for this change.)

ENV/super: fix set_cpu_flags stub.

It should accept arguments.

Fixes https://github.com/Homebrew/homebrew-core/issues/8362

keg: handle dependencies of moved/renamed formulae

Remove `:bsexec` parameter from `SystemCommand`.

In #1497 I switched from Keg#to_formula for comparing kegs to formulae
to comparing the name and tap in the keg's tab to the name and tap of
the formula.

However, this fails to match if the name and tap of the formula have
changed since the keg was installed, so it's clearly better to use
Keg#to_formula where possible, and fall back to the information in the
tab when #to_formula can't be used.

formula, ARGV: don't output duplicate formulae.

keg: fall back for dependencies of buggy tabs

keg: don't return nil dependencies

keg: fix fallback dependencies with multiple kegs

If you specify a formula more than once or it exists in the Cellar with
an alias name and the main name (e.g. `qt` and `qt5`) you can see the
same formula showing up more than once. Instead, resolve these output
lists of formulae such that they are unique based on their `name`. This
doesn't use `full_name` as it's `name` that's use for the `Cellar`.

Return `opt_prefix` if it exists and `prefix` is not called from within
the same formula's `install` or `post_install` methods. Otherwise, fall
back to the existing functionality.

This avoids the need to use `opt_prefix` etc. everywhere and generally
means we don't expose an implementation detail (i.e. the full Cellar
path) to dependents that have a habit of hard-coding it.

This is a proper fix to the problem addressed by #1510.

The problem arises when f_kegs is nil, which can happen if the name and
tap used to install a keg don't match the name and tap currently
associated with its formula (i.e. if it's been renamed or moved).