Move resolving into dedicated plugins

[ef4]

Supersedes #1311.

While this refactor is landing a webpack plugin, I have already tested that the same shared module-resolver API is fully working under rollup and vite.

[ef4]

Goals as we land this:

• move all app tree merging into the module-resolver rules, so no physical rewriting of the app is necessary
• redirect resolution to and from auto-upgraded addons so we don't need to physically recreate the node_modules tree (this would simplify the existing case where v2 addons depend on emberVirtualPeerDeps)
• refactor class CompatResolver into separate steps: the first would be a one-to-one transformation of the template that always emits standardized imports, the second would be additional rules in the module-resolver that satisfies those imports based on what's actually available.

[ef4]

The unchecked "goals" list above is things to do in the next PR.