Don't unpack sources into X-extracted packages

[wmertens]

This slows down builds some with all the bookkeeping, and simply unpacking every time they're needed should be faster.

If there are builders that are faster thanks to this, perhaps only they should do this?

[DavHau]

Currently our sourceSpec supports a dir argument which allows to specify a subdir of a source to be the actual source. Currently this directory path is calculated at eval time which only works if all fetched sources are always unpacked by default. Maybe that was a bad design choice after all and should be changed.

We could instead use sourceRoot of mkDerivation, so the subdir path is calculated at build time, not at eval time.

[DavHau]

For now you can let your builder access the original fetched file via the .original attrbiute of the source returned by getSource.
Though it doesn't always exist. This should work:
let src = getSource {name} {version}; in src.original or src