uses: properly handle untapped formulae in recursive dependency expansion

PerlRequirement: Perl version string may lack parentheses

Whitelist PHP formulas in shadowed header audit check

Revert "cc: always filter flags on deps."

keg: use resolved_path not realpath.

This guards against this being a non-symlink in which case it will fail.

Fix abv and prefix_linked?

Use docs.brew.sh links.

formula: correct grammar in comments

docs: fix titles on docs.brew.sh

licenses: update copyright year

For example:
This is perl, v5.10.1 (*) built for x86_64-linux-thread-multi

Fixes #1776.

If any known formula had a dependency on an untapped tap,
Formula#recursive_dependencies would throw an exception, which would be
caught by the outer exception handler, causing the rest of the
dependencies for that formula to be skipped and incomplete output to be
generated.

To fix this, I added a check to avoid analysing the dependencies of
formulae from uninstalled taps.

Additionally, I removed the aforementioned outer exception handler added
in 5fdb89ae, because the only other
place that should be capable of throwing such an exception is the
statement that was surrounded by another wider exception handler in
Homebrew/legacy-homebrew#40682.

Previously, this returned a String, but a Tap instance seems much more
sensible.

I couldn't find anywhere this method was actually used, so the change
shouldn't break anything.

otherwise whenever prefix is unversioned, prefix_Linked will be false

versioned prefix is a directory, but unversioned is a symlink, so
realpath it before computing abv

tests: remove temporary tab homebrew_version override

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 had to be added in #1750 to work around special-casing for tabs
generated with Homebrew versions < 1.1.6. Now that 1.1.6 is the current
version, we can remove this hack.

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.

See https://github.com/Homebrew/brew/pull/1771#issuecomment-270143292.

See https://github.com/Homebrew/brew/pull/1771#issuecomment-270179479.

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