This resolves a regression introduced in fc01353703, where providing a
name without a proper extension breaks existing uses of fetchzip (they
now fail to unpack). Of particular note, that commit broke all uses of
fetchFromGitHub because it uses a name like so: "${repo}-${rev}-src"
Fixes #5954
This fixes:
* Passing stripRoot.
* Archives containing a single file.
* Archives where the root folder has the same name as one of it's children.
Fixes #5851
If $src refers to a directory, then always copy it. Previously, we
checked the extension first, so if the directory had an extension like
.tar, unpackPhase would fail.
This function downloads and unpacks a file in one fixed-output
derivation. This is primarily useful for dynamically generated zip
files, such as GitHub's /archive URLs, where the unpacked content of
the zip file doesn't change, but the zip file itself may (e.g. due to
minor changes in the compression algorithm, or changes in timestamps).
Fetchzip is implemented by extending fetchurl with a "postFetch" hook
that is executed after the file has been downloaded. This hook can
thus perform arbitrary checks or transformations on the downloaded
file.
