Fix line numbers in imports

[mstoykov]

Because of the way we wrap imports, we add an additional 1 to all line numbers. (this is not true for the main script because it special ... for now). This is somewhat annoying and definitely problematic in big scripts when the error is cryptic to begin with.
We should be able to utilize sourcemaps given that goja supports them and we already have some code that reads them even though they were disabled and I can't find them ever being used ... ;(

[na--]

Closing this in favor of #1804, since it's more detailed