Resolve "`git lb-checkout` should allow only check out of _packages_"

• git-lb-checkout
  • allow only packages and hats
  • suggestions for misspelled branch or package name
  • --list option to list packages in a branch/commit
• git-lb-push
  • fix missing import
• git-lb-use
  • fix project case for remote name

Closes #3 (closed)