add `--plugin-conflict-error` to `bw test`
refactor plugin local modification check
use PluginManager.local_modifications() in plugin update
catch PluginLocalConflict when installing a new plugin
remove unused import
add plugin_conflict_error to legacy tests
add tests for plugin conflicts in `bw test`
Merge branch 'feature/test-plugin-conflict-error'

Conflicts:
	tests/unit/cmdline/test_tests.py