ExternalProject: Report error if local variables are not defined

Since in some situations, ExternalProject module may be included in a sub-directory, functions will be available in the global scope but local variables like _ep_keywords_<keyword> will not be defined. This merge request checks and reports an error indicating that the ExternalProject module must be explicitly included before using any of the ExternalProject_* functions that require the module's inclusion within the current scope or above.

Topic-rename: ExternalProject-check-explicit-include