feature: allow alias to submodule paths

[lukeed]

It can be useful to alias framework/entry to other submodules within the framework package. I have a CLI that accepts flags that change what /entry points to & cannot use the new alias option to achieve this.

Right now, esbuild throws this error if you attempt to do that:

Error: Build failed with 1 error:
error: Invalid alias name: "framework/entry"

The workaround (previous approach) is to alias the entry via a plugin's resolve hook.