- Run `mkdirp` on `spec.prefix`
- Extract directly into `spec.prefix`
1. No need for `$store/tmp.xxx` where we extract the tarball directly, pray that it has one subdir `<name>-<version>-<hash>`, and then `rm -rf` the package prefix followed by `mv`.
2. No need to clean up this temp dir in `spack clean`.
3. Instead figure out package directory prefix from the tarball contents, and strip the tarinfo entries accordingly (kinda like tar --strip-components but more strict)
- Set package dir permissions
- Don't error during error handling when files cannot removed
- No need to "enrich" spec.json with this tarball-toplevel-path
After this PR, we can in fact tarball packages relative to `/` instead of `spec.prefix/..`, which makes it possible to use Spack tarballs as container layers, where relocation is impossible, and rootfs tarballs are expected.
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