CMakeLists.txt: Provide both 'freetype' and 'Freetype::Freetype' targets.

FreeType can be located by consuming projects that use
`find_package(Freetype)` either via the old `MODULE` path (which uses
`FindFreetype.cmake` supplied by CMake), or via the new `CONFIG` path (which
uses `freetype-config.cmake` as supplied by this project).  Up to this point
the CMake module has supplied the target `Freetype::Freetype` and the config
file provided by this project the target `freetype`.  Now we supply both
`freetype` and `Freetype::Freetype` so that consuming projects can always
use the target `Freetype::Freetype` regardless of what path was taken by
`find_package(Freetype)`.

Fixes #1165.
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