CMake find_package dependency backend

Meson can now use the CMake find_package ecosystem to detect dependencies. Both the old-style <NAME>_LIBRARIES variables as well as imported targets are supported. Meson can automatically guess the correct CMake target in most cases but it is also possible to manually specify a target with the modules property.

# Implicitly uses CMake as a fallback and guesses a target
dep1 = dependency('KF5TextEditor')

# Manually specify one or more CMake targets to use
dep2 = dependency('ZLIB', method : 'cmake', modules : ['ZLIB::ZLIB'])

CMake is automatically used after pkg-config fails when no method (or auto) was provided in the dependency options.