This is currently still very messy and we probably want to work out the
best way to declare what parts of our DSL/what files are "public".

Still, even if this is a WIP I'd rather get something committed sooner
rather than later and start iterating on this as a replacement for
`example_formula.rb` and the formula cookbook.

To test:
```bash
cd $(brew --prefix)/Library/Homebrew && \
  rdoc formula.rb requirement.rb utils.rb &&\
  open doc/index.html
```

Closes Homebrew/homebrew#32470.

There is a nasty circular dependency here:

formula.rb
  requirement.rb
    extend/ENV.rb
      extend/ENV/shared.rb
        formula.rb

Probably the information that the build environment needs from formula
should be passed in at runtime, but that seems hard and I really just
want to be able to run the tests with warnings turned on. :/

Allows disabling bottles from requirements.

Instead we use which with a custom PATH.

The implementation of #eql? and #hash should ensure that if a.eql?(b),
then a.hash == b.hash, but #eql? itself should not *depend* on #hash.

For example, given

  class Thingy
    def eql?
      instance_of?(other.class) && hash == other.hash
    end

    def hash
      [name, *tags].hash
    end
  end

if #hash produces a collision for different values of [name, *tags], two
Thingy objects will appear to be eql?, even though this is not the case.
Instead, #eql? should depend on the equality of name and tags directly.

This hack was necessary since requirements were not checked again
in the forked build process, but now they are, and calling it again
after the build environment has been set up can produce incorrect
results. In fact, if it happens to return false the second time,
the env modification will be skipped altogether.

Closes Homebrew/homebrew#20239.

Fixes Homebrew/homebrew#19857.

This allows default resolution of requirements without user intervention.

Closes Homebrew/homebrew#19627.

By always passing around a single, unnested array rather than splatting
and then defensively flattening and compacting things, we can avoid
allocating a bunch of unnecessary arrays. This gives a performance boost
of roughly 4% when enumerating 2500 formulae, and has the side effect of
cleaning up the dependency API.

This was meant to support:

  env do |req|
    append_path 'PATH', req.some_method
    ...
  end

i.e., the block was evaluated in the context of ENV. But it turned out
to be not so useful after all, so I'm ripping it out before something
actually depends on it.

The name attribute of requirements is used when generating options for
the :optional and :recommended dependency tags.

Unless otherwise specified, the name attribute of a Requirement will be
populated by stripping any module prefixes from the beginning and
"Dependency" or "Requirement" from end of the class name and downcasing
the result.

Closes Homebrew/homebrew#17759.

Expand requirements recursively while applying the same optional? and
recommended? filters that dependencies are run through. Options
generated by requirements are now checked against the correct list of
requirements, eliminating the temporary "best guess" logic in the
installer.

When a formula's dependency tree contains more than one X11 dependency,
they are de-duplicated by comparing the min_version attribute. However,
this can result in broken dependency trees if one of the X11Dependency
objects was actually specified as e.g. `:libpng`.

In practice, this only matters when one or more of the dependencies has
additional metadata that makes it distinct from the rest, i.e. an
:optional or :recommended tag.

To combat this, make these special, "partial" X11 dependencies instances
of different classes so that they are not de-duped.

It will still be necessary, at the time when requirements are expanded
by the installer, to de-duplicate any remaining X11 dependencies after
applying the optional/recommended filters in order to avoid duplicated
modifications to the environment (as ENV.x11 is not idempotent).

c.f. Homebrew/homebrew#17369.