Allow 'some-type:main' modules to be looked up easier.

This removes the need to define custom resolver methods for every type that
also has a `main` module.

Specifically, in Ember-land we have at least two of these (possibly more):

* `adapter:main`
* `router:main`

Prior to this change you would have to add a `resolveRouter` method to the
resolver. This instructs the resolver to update look for a module with the
`prefix + type`, and falls back on the existing logic if it is not found.

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Please note that this builds off of which has a test to ensure that `router:main` is found when registered as `appkit/router` (with `appkit` as the `modulePrefix`).