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.

Include file at a specific version. Supported options:

version(Version): Include version Version of File, where Version is a gitty commit of the file. This is the same as :- include(Version)., but more explicit.

If the same file is included at two places it is included at most once. Additionally

If neither is versioned the most recent version is included.
If two versions resolve to the same content hash, this is included.
If a specific version is included, subsequent unspecified includes are ignored. A subsequent incompatibly versioned include results in an error.

The envisioned model is that we can specify which version is, possibly indirectly, included by using directives like this:

:- include(File, [version(Hash)]).

Fetch the data to be included and obtain the URI for it.

Normalise an include file identifier and verify its safeness.

Translate Alias/Name into Alias(Name) if Alias is known and Name is safe.

Add a file name extension to indicate this is a Prolog file.

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.

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