Speed up git clones and catalog command

[brikis98]

Description

This PR started as a way to improve the speed of the terragrunt catalog command. However, I believe the improvements here may also speed up Terragrunt in general. Here's a summary of the changes:

1. Add depth=1 parameter to source URLs. This will have go-getter do a shallow git clone. Whenever we clone a repo, it is for read-only usage on a single branch/ref anyway, so a shallow clone should work fine, and should be much faster, especially for larger/older repos.
2. Run the git clone steps of the catalog command concurrently using goroutines. So instead of cloning and parsing one repo at a time, we do them all in parallel (up to number of CPU cores), which speeds things up quite a bit as well.

I tested the catalog command with 4 URLs in my terragrunt.hcl config and the average time for first run when from ~20 seconds to ~5 seconds.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Migration Guide

[brikis98]

Ah, the scaffold command tries to look up Git tags on the repo, so shallow clone doesn't work... Will have to think on this a bit more.