[suggest] Provide `<pkg>::<comp>` Imported Targets with `pango::` namespace.

[hwhsu1231]

For example:

• pango::core
• pango::display
• pango::geometry
• pango::glgeometry
• pango::image
• pango::opengl
• ...etc

While the syntax :: seems to be more modern and conventional in CMake, it has an obvious benefit.

If we try to use an IMPORTED target pango::display but forget to add it as a COMPONENT in find_package(). (BTW, I also hope that Pangolin introduce this kind of usage, like Qt, Boost, and Poco...etc)

find_package(Pangolin REQUIRED COMPONENTS core)
target_link_libraries(myEXE pango::core pango::display)

Then, CMake will report a FATAL_ERROR message according to the policy CMP0028.

Target "myExe" links to target "pango::display" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?