Create proxy classes for "partial" X11 dependencies

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).
wip: requirement expander
Update documentation for Dependency.expand