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.

Closes Homebrew/homebrew#17317.

Use versions to better get hold of the filename.

Use ENV['HOMEBREW_SOURCEFORGE_MIRROR'].

Closes Homebrew/homebrew#15642.
Closes Homebrew/homebrew#17368.

Better version of ed357d.

References Homebrew/homebrew#15711.
References Homebrew/homebrew#17361.
References Homebrew/homebrew#17437.

Also, move the line containing `info`, `home` and `options` to the top of the
list as users should be running these before installation.

Ref. Homebrew/homebrew#13224.

Fixes Homebrew/homebrew#17434.

c.f. Homebrew/homebrew#17361.

Fixes Homebrew/homebrew#17422.

Closes Homebrew/homebrew#13349.

Closes Homebrew/homebrew#17181.

Allows a formula to selectively disable bottle pouring.
e.g. the default Boost bottle does not work with a brewed Python.

Fixes Homebrew/homebrew#17142

Closes Homebrew/homebrew#14270

Closes Homebrew/homebrew#15711.

Gmock and GTest should not be installed system-wide.

Closes Homebrew/homebrew#17358.

Revert "Prevent linkapps from creating duplicate aliases"
Revert "linkapp: create Finder aliases"

This reverts commit e1a68f1ffbac9dc8209cc9ac2c8f2e34d11867af.
This reverts commit bf2e6023d1bac08739b4562e19e07db448f0aba3.

Closes Homebrew/homebrew#15308.

Fixes Homebrew/homebrew#17403.

Closes Homebrew/homebrew#17383. Closes Homebrew/homebrew#17384.

Fixes Homebrew/homebrew#17379.