package: removed superfluous character.
modules: updated FindCorrade.cmake and FindMagnum.cmake.
Install plugins in debug build to different location.

Makes it possible to have both debug and release plugins installed. If
both debug and release libraries are present when finding the package,
proper version is used based on what configuration is used in depending
project.
package: added release PKGBUILD.

It compiles and installs both debug and release libraries.
doc: removed info about internal CMake variables.

Keep the docs only in FindMagnumPlugins.cmake.