Allow mocking and aliasing externals

[pi0]

When bundling and inlining dependencies, we have a chance to use rollup configuration to mock some unwanted external dependencies using unenv (nuxt uses this for example for vue deps).

This is not possible when externalizing those dependencies and using nitro externals + nft to track them.

With a new option like externals.alias, we can instruct nft to use a mocked and auto-generated package instead. Previously it wasn't possible but with new implementation of externals plugin with full-control over dep graph, we can do this.