Fixes "No such file or directory" error when running prefetch-npm-deps
in its default mode (print hashes, for update scripts etc),
in an environment that doesn't have a `nix` binary (for `nix hash`).
GNU tar will apparently silently include mtime of files if --mtime is
passed with an unrecognized date format. This led to hash instability
from those mtimes and this fixes it to force all mtimes to epoch
timestamp 0.
Previously, we stored the tarballs from the hosted Git providers directly in the cache. However, as we've seen with `fetchFromGitHub` etc, these files may change subtly.
Given this, this commit repacks the dependencies before storing them in the cache.
A `package-lock.json` file can contain multiple instances of the same dependency, which caused unnecessary downloads and duplicate index entries in the generated cache.
