Just to clarify: the source tarball are *never* modified to specifically
make sure hashes don't randomly change (which would trip all sorts of
warnings for distribution packagers). In fact, there's also a
`cabal get` option if you want to unpack with the original unaltered .cabal
file:

    --pristine    Unpack the original pristine tarball,
                  rather than updating the .cabal file
                  with the latest revision from the
                  package archive.