virtual/imagemagick-tools: Add virtual for media-gfx/{image,graphics}magick

* Packages that do **not** link to either library and just consume the
  runtime command-line tools of either package should depend on the virtual
  in general. This reduces the complexity of the depgraph and makes central
  USE flag handling much easier.
* Packages that can link to either imagemagick or graphicsmagick need more
  delicate handling in order to be usable with subslots.
  1) Packages that **require** either imagemagick or graphicsmagick should
     use USE="graphicsmagick" to differentiate which one to use by specifying

	!graphicsmagick? ( media-gfx/imagemagick:= )
	graphicsmagick? ( media-gfx/graphicsmagick:= )

  2) Packages that **may** use either imagemagick or graphicsmagick, but don't
     **require** it, should additionally add USE="imagemagick" and specify

	imagemagick? (
		!graphicsmagick? ( media-gfx/imagemagick:= )
		graphicsmagick? ( media-gfx/graphicsmagick:= )
	)

  So that the semantics become:

    USE="imagemagick"     = "I want to build with optional imagemagick or graphicsmagick support"
    USE="-imagemagick"    = "I do NOT want optional support for imagemagick and/or graphicsmagick"

    USE="-graphicsmagick" = "I want to build against media-gfx/imagemagick"
    USE="graphicsmagick"  = "I want to build against media-gfx/graphicsmagick"

  This avoids package.use pollution due to setting REQUIRED_USE.

Bug: https://bugs.gentoo.org/show_bug.cgi?id=314431
Package-Manager: Portage-2.3.3, Repoman-2.3.1
Closes: https://github.com/gentoo/gentoo/pull/3907

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