ADDED: in-buffer completions for source file specs

ADDED: in-buffer completions for source file specs

* sweep.pl (sweep_alias_source_file_name_collection/2)
(sweep_alias_source_file_name/4)
(sweep_source_file_name_collection/2)
(sweep_source_file_name/3)
(sweep_path_alias_collection/2)
(sweep_matching_path_alias/3): New predicates.
(sweep_context_callable_/3): Simplify, and recognize predicate
arguments that should be source file specifications.

* sweeprolog.el (sweeprolog-source-functor-completion-candidates)
(sweeprolog-alias-source-completion-candidates)
(sweeprolog-source-completion-candidates): New functions, used in...
(sweeprolog--atom-or-functor-completion-at-point): ...here.
(sweeprolog-electric-layout-post-self-insert-function)
(sweeprolog-context-menu-for-region)
(sweeprolog-maybe-extract-region-to-predicate): Adapt.

* sweep.texi (Code Completion): Document file spec completion.
* sweeprolog-tests.el: Test it.