fix: resolve flake8 E305/E402 lint errors in plugin loader

Move imports to top of file and ensure 2 blank lines after function
definition to satisfy flake8 rules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wu-sheng

skywalking/plugins/__init__.py

@@ -22,12 +22,6 @@
 import re
 import traceback
 
-PackageNotFoundException = importlib.metadata.PackageNotFoundError
-
-
-def get_pkg_version(pkg_name):
-    return importlib.metadata.version(pkg_name)
-
 from packaging import version
 
 import skywalking
@@ -36,6 +30,12 @@ def get_pkg_version(pkg_name):
 from skywalking.utils.comparator import operators
 from skywalking.utils.exception import VersionRuleException
 
+PackageNotFoundException = importlib.metadata.PackageNotFoundError
+
+
+def get_pkg_version(pkg_name):
+    return importlib.metadata.version(pkg_name)
+
 
 def install():
     disable_patterns = config.agent_disable_plugins