JSR exports not added to importmap

[zefhemel]

Deno supports the following: when you put an exports section in your deno.json file as well as a package name, this implicitly adds an entry to your import map for your exports, so that you can simply do import ... from "packagename".

This does not yet work for this loader, so I have to duplicate all exports into my imports section now. Ideally, the ESBuild Deno loader would be consistent with Deno behavior and do this for me.