Adds basic reachability integration in cli (Part 1/N)

[meghfossa]

Overview

This is part of stack PR. This PR will not be merged, until all stacked PR's are merged into this branch.

This PR,

• Adds basic command line tool for reachability (reachability/extlib)
• Adds hidden fossa reachability <PATH> command for debugging

Acceptance criteria

• Automated build works
• fossa reachability <PATH> can be executed (It will fail in part 1 PR)

Testing plan

git checkout master && git pull origin && git checkout feat/reachability-cli && make install-dev

after which,

fossa-dev reachability

# Note that this command will fail, as this PR does not add implementation
# so error is to be expected! You should see something like following in error message:
#
# Command error log:
#            DEBUG [reachability] Reading from file system: "/Users/megh/code/fossa-cli/"
#            DEBUG [reachability] Parsing file system: "/Users/megh/code/fossa-cli/"
#            thread 'main' panicked at extlib/reachability/src/lib.rs:31:9:
#            not yet implemented
#            note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Risks

N/A

Metrics

N/A

References

N/A

Checklist

• I added tests for this PR's change (or explained in the PR description why tests don't make sense).
• If this PR introduced a user-visible change, I added documentation into docs/.
• If this PR added docs, I added links as appropriate to the user manual's ToC in docs/README.ms and gave consideration to how discoverable or not my documentation is.
• If this change is externally visible, I updated Changelog.md. If this PR did not mark a release, I added my changes into an # Unreleased section at the top.
• If I made changes to .fossa.yml or fossa-deps.{json.yml}, I updated docs/references/files/*.schema.json AND I have updated example files used by fossa init command. You may also need to update these if you have added/removed new dependency type (e.g. pip) or analysis target type (e.g. poetry).
• If I made changes to a subcommand's options, I updated docs/references/subcommands/<subcommand>.md.

[meghfossa]

I have intentionally left out implementation. This will be added in second PR tomorrow.

[spatten]

I have one question about the proliferation about error handling libraries that we use, but I don't think that's blocking.

Other than that, looks great!

[spatten]

I'm worried that we are using too many error handling libraries.

Broker uses error-stack. Lernie and Sherlock use this-error. Reachability-toolkit uses anyhow.

Now we're adding eyre into the mix.

If there's a good reason to use stable-eyre instead of one of the other three, then that's fine. But I wanted to double-check

[meghfossa]

I copied the dependencies and code structure from berkelydb, but I can modify it to recent rust code one's.

[spatten]

Hmm. Maybe we should merge this as-is, but have a quick conversation about whether we want to standardize this a bit in standup tomorrow.

I don't think it blocks this PR; we can just make the change later if we decide to

[meghfossa]

Superseded by: #1372