Don’t unnecessarily invalidate the module map (#124)

Don’t unnecessarily invalidate the module map (#124)

* Don’t unnecessarily invalidate the module map

Additional imports in PureScript sources (compared to their JavaScript output) shouldn’t always invalidate the module map because imports of types are erased and re-exports are followed.

Also `Prim.*` modules are internal to the compiler and won’t ever be present in the module map.

* Don’t add imports to unused modules

Otherwise the following expression

```purs
hello :: Effect Unit
hello = log "Hello"
```

includes the whole `Prelude` into its chunk whereas only `log` is actually needed at runtime.