refactor: centralise imports and cleanup exports

[iuioiua]

The benefits of these changes are:

1. Managing and updating dependencies is easier. One only needs to run deno outdated at the root, instead of in each workspace.
2. The versions of dependencies across workspaces is kept consistent.

Closes #2788

[iuioiua]

PTAL @marvinhagemeister

[marvinhagemeister]

I fee like this PR is a good argument why Deno should support workspace: outside of package.json. It's perfectly reasonable to centralise versioning of shared dependencies in the root deno.json file like done here, but you still want to be able to see what the dependencies of each package are. I'll bring it up with the CLI team.

[iuioiua]

We could just use deno.jsonc.

[marvinhagemeister]

We could just use deno.jsonc.

I'm not sure I'm following. What would change and what it would look like?

[iuioiua]

Imports within deno.jsonc sectioned off by comments indicating the workspace the group of imports is for. Happy to implement.

[iuioiua]

@marvinhagemeister, is not being able to know which dependencies each workspace uses a blocker for this PR? If so, I can rename the deno.json to deno.jsonc and section off each group of imports by workspace using a comment. But that'd need other small changes to the codebase that read the root config file. This PR would cure quite a few ills, including potential ones.

[marvinhagemeister]

LGTM

is not being able to know which dependencies each workspace uses a blocker for this PR?

No, I've changed my mind on this one since the recent issue where due to missing peerDependencies I published an alpha with duplicate Preact versions. Let's merge this.