CLI: Improve keymap folder resolution

[fauxpark]

Description

• Renamed qmk.path.keymap() to keymaps() and changed return value to a list of all found keymaps/ directories for the given keyboard
• keymap() now takes a keyboard name and keymap name, and returns the path if it exists

This fixes an issue with qmk new-keymap: for example, attempting to create a new keymap for a board such as crkbd/rev1 fails as there is no default keymap under rev1's keymaps/ directory. Now it should grab the keymap folder from parent level and copy it down.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).