Implement 'wkg get'

[lann]

$ wkg get wasi:http
No version specified; fetching version list...
Getting wasi:[email protected]...
Wrote './[email protected]'
$ wkg get --registry wow.wa.dev wow:[email protected]
Getting wow:[email protected]...
Wrote './[email protected]'

$ wkg get --help
Get a package

Usage: wkg get [OPTIONS] <PACKAGE_SPEC>

Arguments:
  <PACKAGE_SPEC>  The package to get, specified as <namespace>:<name> plus optional @<version>, e.g.
                  "wasi:cli" or "wasi:[email protected]"

Options:
  -o, --output <OUTPUT>    Output path. If this ends with a '/', a filename based on the package
                           name, version, and format will be appended, e.g.
                           `[email protected]`` [default: ./]
      --format <FORMAT>    Output format. The default of "auto" detects the format based on the
                           output filename or package contents [default: auto] [possible values:
                           auto, wasm, wit]
      --overwrite          Overwrite any existing output file
      --registry <DOMAIN>  The registry domain to use. Overrides configuration file(s)
  -h, --help               Print help