Improve fetching of go and cargo dependencies

[climbfuji]

Is your feature request related to a problem?

We currently maintain two utility scripts util/fetch_cargo_deps.py and util/fetch_go_deps.py to fetch Cargo and Go dependencies for an active environment and store them in separate mirrors (defined by environment variables).

These scripts mostly work well, but at times the mirrors, especially the Cargo mirror - needs to be removed and rebuilt from scratch to fix Cargo dependency and/or download errors.

Also, it was discussed in the past that we turn these tools into spack extensions, for example spack stack fetch-go-deps.

Describe the solution you'd like

1. Create spack stack extensions of the spack command to fetch Cargo and Go dependencies en lieu of the current utility scripts.
2. Remove the utility scripts and update any automated tools (weekly builds, NRL batch install, ...) as needed.
3. Improve the error handling of these extensions to reduce the number of times spack-stack users have to manually remove the Cargo or Go mirrors and rebuild them from scratch.

Additional context

We've identified this to be a part of #1772