Automatically detect package aliases

Context of the issue: https://github.com/unjs/nitro/pull/1553

When tracing external packages, pnpm uses symlinks to the aliases package name from expected name that makes tracing aliases virtually impossible.

Currently, it is possible to specify aliases directly in config:

import { defineNitroConfig } from "nitropack/config";

export default defineNitroConfig({
  externals: {
    traceAlias: {
      "@sxzz/popperjs-es": "@popperjs/core",
    },
  },
});

Since we have access to the project package.json and tracked dependencies, we might make it automated (to an extend). One possible limitation is when more than one possible alias target is discovered.

Start by reading the linked pull request and the externals.traceAlias configuration shown in the issue, then inspect how the project package.json and tracked dependencies are used for external-package tracing. Define how an alias is detected and how multiple possible targets are handled; done means supported aliases are detected automatically without requiring equivalent manual configuration.