Enable user-defined namespacing for packages in gren.json

From the module conflict resolution thread on Zulip:

To mitigate the problem of module name clashes between multiple dependencies, or between a dependency and a project's own source code, what if we could optionally define a namespace in gren.json for each imported package, e.g.:

"dependencies": {
    "direct": {
        "justinmimbs/date": {"version": "3.2.1", "namespace": "Internal"},
        ...

Then we could use import Date to import a local Date.gren module from our own project's src folder, while import Internal.Date would refer to the one from the justinmimbs/date package.

Another, possibly better, option is to allow imports in each file to specify the package they are imported from, something like:

import Date from justinmimbs/date
import Date from someoneelse/date as Date2

Start with the linked module conflict resolution thread, then trace how gren.json dependencies and source-file imports are parsed and resolved. The proposal presents two alternative designs, so first establish which behavior is wanted; done should include a decided namespace or package-qualified import model with corresponding compiler coverage.