feat: Expand 'imports' section of deno.json

[bartlomieju]

This commit adds automatic expansion of "imports" field in "deno.json" file.

If "npm:" or "jsr:" imports are encountered we automatically try to add a "directory"
remapping.

Previously users had to specify entries for both foo and foo/ to be able to import like
import { symbol1 } from "foo"; and import { symbol2 } from "foo/some_file.js":

{
  "imports": {
    "foo": "npm:@foo/bar",
    "foo/": "npm:/@foo/bar/",
}

With this change users can only add entry for foo:

{
  "imports": {
    "foo": "npm:@foo/bar",
}

The entry for foo/ will be provided automatically.

Similarly if user provides "directory" remapping explicitly, we will not overwrite it.