migrate source-tarball-unpacking to library, use in registry api

[syphar]

for my docs_rs_build CLI (to test your crate builds in CI), I need to unpack the source tarball that I generated with cargo package. Since I don't want to duplicate that functionality, I now

• move just the source tarball logic into a separate new crate docs_rs_crate_archive
• add download_and_extract_source to the registry API, including the extension to TestRegistry to mock the download response.
• make the import_release CLI use that download_and_extract_source API.

Later I can then just use docs_rs_crate_archive functionality to unpack the cargo package output.

[syphar]

r? @GuillaumeGomez