Extend 'qmk info' to handle keymap level overrides

[zvecr]

Description

Partially cherry-picked from #13733, but extended to qmk info.

Examples

qmk info -f json -kb durgod/dgk6x/hades -km via | grep -A2 dynamic
    "dynamic_keymap": {
        "eeprom_max_addr": 4095,
        "layer_count": 16

qmk info -f json -kb thevankeyboards/minivan -km via | grep -A2 dynamic
    "dynamic_keymap": {
        "layer_count": 4
    },

Future TODOs?

• Handle config.h/rules.mk in userspace?
• Handle config.h/rules.mk at keymap level when also processing community layouts

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: 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).