include.pl -- Support :- include(File) from SWISH

This module allows SWISH programs to include other programs from the shared gitty store. It realises this using the following steps:

• Use term_expansion/2 to rewrite the include to fetch the data from the gitty store.
• Declare this specific version of include safe.
• Adjust the colourization to indicate the shared file as existing.
• Hook the Prolog cross-referencer to process the included file.

We allow for hierarchical and circular includes.

 prolog:xref_source_identifier(+Src, -Id) is semidet[multifile]

prolog:xref_open_source(+File, -Stream) is det[multifile]

prolog:xref_source_time(+File, -Modified) is det[multifile]

Map swish://file to a file from the gitty store.

 prolog:xref_source_file(+Term, -Path, +Options)[multifile]

Deal with the above expansion for :- include(program) to support the cross-referencer.