Packages like `alex` include pre-generated lexer/parser source
in their source tarball as e.g.

    dist/build/alex/alex-tmp/Scan.hs
    dist/build/alex/alex-tmp/Parser.hs

to avoid having to have `alex` already installed before building
`alex`...

Unfortunately, 8dc39db7 broke
packages relying on this accidental feature by changing where the
distdir points to. This patch partly reverts that commit and
therefore addresses the regression aspect of #4009.

This fix was suggested by @dcoutts