Replace forum and admin global compat exports with a Proxy

[dsevillamartin]

Refs #2085.

Replaces flarum/flarum-webpack-config#7.

Adds ability for import statements of core & core extensions to include their namespaces (forum, admin, and common).

We'll eventually require extensions use the namespaces in their imports. This PR makes them optional for extensions and allows extensions to continue working once we require them.

Original compat objects are left intact & do not contain the namespaces.

• import admin/utils/extract --> returns import utils/extract
• import tags/common/utils/sortTags --> returns import tags/utils/sortTags

Technically extensions can also use the incorrect namespace (forum/admin and common) as this code doesn't check whether it's correct. This shouldn't be a problem that we focus on, however.

[askvortsov1]

Seems to work when tested locally, makes sense to me! Do we want to add a deprecation warning for the old system? Or let these exist in parallel before deprecating the old one? Maybe next release we should sweep through and deprecate all the old systems (including the @flarum/namespace/path one)?

[dsevillamartin]

Do we want to add a deprecation warning for the old system?

What would this deprecation warning look like? We'd only want to log one message because if we do every deprecated import, the console would be flooded with these messages.

Or let these exist in parallel before deprecating the old one?

Yeah I think we should deprecate these in the next release.

Maybe next release we should sweep through and deprecate all the old systems (including the @flarum/namespace/path one)?

Yes, though I think the @flarum/core is for public API so we wouldn't want to do that? Still not entirely sure.