uses: fix recursive requirement resolution

Add `keg_only :versioned_formula`.

suite_test: skip flaky test.

This is used to indicate a formula is a version of another formula.
This will be used to provide a consistent interface for older formulae
versions and replaces the use of `conflicts_with`.

requirement: get formula from satisfy.

audit: don't allow universal for new formulae.

create: handle null versions.

Unset GREP_OPTIONS.

tests: test Formula#recursive_requirements

Fixes #1848 by sharing recursive dependency resolution between
dependencies and requirements.

Coincidentally, this also fixes the errors introduced by #1784 that
necessitated it being reverted in #1797.

Users may have this set, breaking grep's output.

Fixes #1430.

clarify what should be in brew create url

Fixes #1821

New feature: Allow download from private GitHub repository

When a Requirement is converted to a Formula it should be listed under
the runtime dependencies.

brew.1: use uninstall instead of remove.

Fix documentation for HEAD upgrades.

formula_installer: set specified_path on pour.

This means that a `brew install $ALIAS` records the path accordingly
so  e.g. `Formula#full_installed_specified_name` returns the correct
path.

We're frowning on these now so may as well turn that into code.

These can now be done with just `brew upgrade`, like other upgrades.

Fixes #1818.

audit: whitelist unstable versions already in core

also allow higher stable versions with the same version prefix

resolves https://github.com/Homebrew/brew/issues/1821

InreplaceError: fix undefined method crash

brew deps --tree: fix gap in line between reqs and deps

When the first parameter to inreplace was an array, and the replacement
failed, InreplaceError would end up crashing with an undefined method
exception because the order of operations resulted in super not being
passed the value of the entire inject block.

Fixes #1817.

If satisfy returns a `Pathname` from `which` then we can use that to
infer a formula dependency from that `Requirement`.

Deprecate brew (un)linkapps.

xcode, xquartz: use default location when possible.

formula: return runtime_dependencies in to_hash.

formulary: read from formula in opt prefix.

Xcode can be installed anywhere but for most people it's in
`/Applications/Xcode.app` so just look there if `xcode-select` isn't
helpful before looking at Spotlight which can return weird results on
e.g. backup disks.

Fixes #1587.