Rationalisation of dump files

[RichardWallis]

Simplified and expanded dump files and updated associated documentation.

Results in:

• schema.* files now contain all terms.
• all-layers.* are duplicates of these updated schema.* files
• updates to docs/developers.html to reflect these changes

Implements issue #2304

Note: The associated tweaks to the documentation assumes this PR will be merged and shipped in V9.0. If it does not make it into V9.0, there will be a need for a very minor tweak to the wording.

[danbri]

My review comments are in #2304

[RichardWallis]

Following comments on issue, this PR has been updated to reflect the following file name changes and contents:

• schemaorg-current - contains all terms from all sections
• schemaorg-all - contains all terms from all sections plus attic

[danbri]

I've suggested to @RichardWallis that we address the desire for https-based URIs in our data dumps here, but adding parallel definitions (for a year or two) that use https spelling.

• schemaorg-current-http
• schemaorg-current-https
• schemaorg-all-http
• schemaorg-all-https

#2516

Regarding mappings between the two, I would be OK with adding OWL https://www.w3.org/TR/owl-ref/#equivalentClass-def and https://www.w3.org/TR/owl-ref/#equivalentProperty-def to the http file, and to a separate http-https-mappings file.ttl for the convenience of those who care. I do not want to burden the general ecosystem with the expectation that this is the right way to do things. It doesn't handle enumeration values, for example. I also do not want to see owl:sameAs used here; there is no reason to be having the debate on whether https://schema.org/Thing and http://schema.org/Thing are the self-same thing or two things with some kind of equivalence. We should show a clear lead that we'll be using https: as our canonical identifiers in the future, and provide utility dumps for those who aren't.