[Feature Request] Exclude external userspace from lint checking

Feature Request Type

• Core functionality
• Add-on hardware support (eg. audio, RGB, OLED screen, etc.)
• Alteration (enhancement/optimization) of existing feature(s)
• New behavior

Description

qmk lint can be run locally before submitting a keyboard PRs to avoid unnecessary churn of commits just to satisfy the the CI lint checker. However, when run locally, external userspace keymaps are included in keymap list, so via[l] keymaps get erroneously flagged as "should not exist" even when they are properly located outside of the qmk_firmware repository. When checking for the presence of those keymaps, it would be ideal if the keymap list did not include keymaps found in the external userspace.