Currently we check if "tag" is present in LANGUAGE_MODULES for every
String dep, even if tag is nil. Stop doing this, and make the
LANGUAGE_MODULES array into a Set instead to improve lookup performance.
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.
ClosesHomebrew/homebrew#17759.
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.
