Add ability to use keymap.json files in layouts/

[firetech]

Add ability to build keymaps from keymap.json in the layouts directory as well.

Description

Currently, keymaps can be built from JSON format when placed in keyboards/.../keymaps/.../, but only keymap.c is supported when placed in layouts/. This small change adds support for keymap.json files placed in the layouts directory.

Types of Changes

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

Checklist

• My code follows the code style of this project.
• 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).

[skullydazed]

Thanks!