Provide a common interface for Dependency and Requirement

Introduce a new Dependable module that provides useful interrogation
methods for Dependency and Requirement. These methods are mapped to
dependency tags in formula (:build, :optional, and :recommended).

Previously only Dependency objects recorded tags; this allows
Requirements to do the same.

Move Requirement subclasses to a separate file.
Formula#recursive_dependencies

This behaves like recursive_deps, but the resulting list consists of
Dependency objects instead of Formula objects. The list maintains the
installable order property of recursive_deps.

While in the area, add some comments clarifying the purpose of related
methods.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Dependency#to_formula and associated helpers

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: disambiguate show_header usage

show_header should default to false (we don't want to display it in the
case of installing a single formula without dependencies), but it only
worked this way by accident.

The assignment "show_header = true" creates a local variable named
show_header, and in the case where needed_deps is empty and this
assignment is not actually executed, the latter usage evaluates the
local variable instead of calling the method.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: factor out requirement checking

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: factor out dependency installation

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: skip build-time deps for bottles

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: implement installation locks

FormulaInstaller now attempts to take a lock on a "foo.brewing" file for
the formula and all of its dependencies before attempting installation.

The lock is an advisory lock implemented using flock(), and as such it
only locks out other processes that attempt to take the lock. It also
means that it is never necessary to manually remove the lock file,
because the lock is not enforced by I/O.

`brew uninstall` learns to respect the lock as well, so that kegs in the
dependency tree of foo can't be removed when another process is
installing foo.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Respect operation lock in link and unlink

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Formula::finalize_dsl

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Add support for optional and recommended deps

Optional deps are not installed by default but generate a corresponding
"with-foo" option for the formula. Recommended deps _are_ installed by
default, and generate a corresponding "without-foo" option.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Let option override with/without descriptions

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Dependency.expand_dependencies

Move Formula.expand_dependencies into the Dependency class, and extend
it to allow arbitrary filters to be applied when enumerating deps.

When supplied with a block, expand_dependencies will yield a [dependent,
dependency] pair for each dependency, allowing callers to filter out
dependencies that may not be applicable or useful in a given situation.

Deps can be skipped by simple calling Dependency.prune in the block,
e.g.:

  Dependency.expand_dependencies do |f, dep|
    Dependency.prune if dep.to_formula.installed?
  end

The return value of the method is filtered list.

If no block is supplied, a default filter that omits optional or
recommended deps based on what the dependent formula has requested is
applied.

Formula#recursive_dependencies is now implemented on top of this,
allowing FormulaInstaller to exact detailed control over what deps are
installed. `brew missing` and `brew upgrade` can learn to use this to
apply the installed options set when expanding dependencies.

Move Formula.expand_deps and Formula#recursive_deps into compat, because
these methods do not respect the new optional and recommended tags and
thus should no longer be used.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: make tab an attr, but never a parameter

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: don't install bottle if options were passed

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
FormulaInstaller: allow formulae to pass options to deps

Formulae can now pass build options to dependencies. The following
syntax is supported:

  depends_on 'foo' => 'with-bar'
  depends_on 'foo' => ['with-bar', 'with-baz']

If a dependency is already installed but lacks the required build
options, an exception is raised. Eventually we may be able to just stash
the existing keg and reinstall it with the combined set of used_options
and passed options, but enabling that is left for another day.
FormulaInstaller: consolidate ARGV munging

There are several steps required to construct the ARGV that is passed to
the spawned build process. Factor them out into a separate method.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
missing: ignore unused optional and recommended deps

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
deps: reimplement intersection with sets

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
uses: utilize modern dependency API

Signed-off-by: Jack Nagel <jacknagel@gmail.com>