Simplify Keg class

This seems to be a remnant of Ruby 1.8.

tests: add a missing Tab cache clear

Similar to 6fc1747e. I hadn't seen any
test failures caused by this, but I noticed it in the process of
trackign that down and it's an accident waiting to happen.

This was responsible for the sporadic failures I encountered in #1878.

tab: store installed_as_dependency, installed_on_request.

Also, report formulae installed on request. This is useful in
differentiating between those formulae that are popular because they
are widely requested and those that are popular because they are widely
depended on.

Which, in turn, provides them for `brew info --json=v1` so other tools
such as e.g. `brew bundle` can make use of this information.

These are used to determine whether or not a formula’s install was
specifically requested by a user and/or pulled in as a dependency.

integration_command_test_case: improve fail output

Instead of `puts`ing when the failure occurs save it until the error
message and print a prose description of what the failure was and the
output from the command. This makes the output from failing tests
significantly easier to read.

bottle: add: improve regexp to recognize comments

audit: only warn on versioned conflicts_with.

uses: properly handle untapped formulae in recursive dependency expansion (again!)

uses: fix recursive requirement resolution

Rather than all versioned formulae regardless. Oops.

This would have eliminated the need for Homebrew/homebrew-core#9000, for
instance.

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

This is a temporary measure until:

- #1862 is merged
- I can a test on `brew uses` against every formula in the official
  taps to verify that exceptions are no longer raised.

…rmulae in recursive dependency expansion""

This reverts commit dc9819b8.

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