Fix qmk find failure due to circular imports

[sigprof]

Description

There was an import cycle in the Python modules:

• qmk.build_targets imported qmk.cli.generate.compilation_database;
• importing qmk.cli.generate.compilation_database requires initializing qmk.cli first;
• the initialization of qmk.cli imported the modules for all CLI commands;
• qmk.cli.compile imported qmk.build_targets.

This cycle did not matter in most cases, because qmk.cli was imported first, and in that case importing qmk.cli.generate.compilation_database did not trigger the initialization of qmk.cli again. However, there was one corner case when qmk.bulld_targets was getting imported first:

• The qmk find command uses the multiprocessing module.
• The multiprocessing module uses the spawn start method on macOS and Windows.
• When the spawn method is used, the child processes initialize without any Python modules loaded, and the required modules are loaded on demand by the pickle module when receiving the serialized objects from the main process.

The result was that the qmk find command did not work properly on macOS (and probably Windows too); it reported exceptions like this:

ImportError: cannot import name 'KeyboardKeymapBuildTarget' from partially initialized module 'qmk.build_targets' (most likely due to a circular import)

Moving the offending qmk.cli.generate.compilation_database import into the method which actually uses it fixes the problem.

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

• qmk find invocations (in particular, qmk find -km all) failed at least on macOS.

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).

[daskygit]

Fixes issue on msys and macos.