RE: Ability to reference other extensions by schema in extension scripts

> 
> "Regina Obe" <lr@pcorp.us> writes:
> >> I have a distinct sense of deja vu here.  I think this idea, or
> >> something isomorphic to it, was previously discussed with some other
> syntax details.
> 
> > I found the old discussion I recalled having and Stephen had suggested
> > using @extschema{'postgis'}@ On this thread --
> > https://www.postgresql.org/message-
> id/20160425232251.GR10850@tamriel.s
> > nowman.net
> > Is that the one you remember?
> 
> Hmmm ... no, ISTM it was considerably more recent than that.
> [ ...digs... ]  Here we go, it was in the discussion around converting
contrib SQL
> functions to new-style:
> 
> https://www.postgresql.org/message-
> id/flat/3395418.1618352794%40sss.pgh.pa.us
> 
> There are a few different ideas bandied around in there.
> Personally I still like the @extschema:extensionname@ option the best,
> though.
> 
> 			regards, tom lane

Here is first version of my patch using the @extschema:extensionname@ syntax
you proposed.

This patch includes:
1) Changes to replace references of @extschema:extensionname@ with the
schema of the required extension
2) Documentation for the feature
3) Tests for the feature.

There is one issue I thought about that is not addressed by this.

If an extension is required by another extension and that required extension
schema is referenced in the extension scripts using the
@extschema:extensionname@ syntax, then ideally we should prevent the
required extension from being relocatable.  This would prevent a user from
accidentally moving the required extension, thus breaking the dependent
extensions.

I didn't add that feature cause I wasn't sure if it was overstepping the
bounds of what should be done, or if we leave it up to the user to just know
better.

Thanks,
Regina