Draft: FindImageMagick: modernize Find-module according to cmake-developer(7) guides

• Add imported targets;
• Update documentation;
• Pass-through compile_definitions from .pc to imported targets and ImageMagick_${component}_FLAGS (?);
• Use FindOpenMP if any related flag (-fopenmp) has been found in .pc flags;
• Create cache variable ImageMagick_ROOT_DIR (according to cmake-developer(7)'s Standard Variable Names (???? there are no way to pass-through it to pkg_check_modules() ???);
• Deal with Windows IMPLIB_LOCATION vs IMPORTED_LOCATION (??????);

Closes #22521 , #15007 , #14568.