Don't import not used commands modules

[mondeja]

As you can see in the next image, when you run mkdocs build, serve, gh_deploy and new modules commands are also imported, although they are not used:

Created with pyinstrument -o out.html -r html -m mkdocs -- build

These imports add some initialization time to the build because some dependencies like watchdog must be initialized. This PR solves it moving the imports to their respective functions.

[oprypin]

Nice!