Feature request: self-extracting extra-source

[kit-ty-kate]

It is sometimes needed to download an extra tarball and extract it somewhere during the build process (e.g. when the tarball doesn't contain the source of a git submodule)
However currently extra-source doesn't allow to extract tarballs contrary to url.
An option could be added like:

extra-source "tarball.tar.gz" {
  src: "path-to-tarball.tar.gz"
  checksum: "xzy"
  extract-to: "vendor/my-vendored-package"
}

The only way to do this currently is to call tar, mv and so manually which is often not very portable and not very user-friendly.

Examples include at least:

• https://github.com/kit-ty-kate/opam-alpha-repository/blob/master/packages/melange/melange.3.0.1-51~alpha-repo/opam
• https://github.com/ocaml/opam-repository/blob/b8624d2cba72498d9335456a36aee84fc7874f94/packages/DkSDKFFIOCaml_StdExport-linux_x86_64/DkSDKFFIOCaml_StdExport-linux_x86_64.1.0.0~1/opam (cc @jonahbeckford)

[dra27]

This approach would be more elegant, but note that both the flexdll and winpthreads already do this slightly more portably by installing the sources instead: e.g. winpthreads.20240209-1.opam uses this .install file.

The downside is that the sources live in the switch, so it's only really viable if the submodule is very small.