Use built-in pkg_resources to get installed libraries

Previously, we used internal pip code, which was changing in point
releases, causing issues. We actually don't need pip to do this, we can
instead ask the installed python's pkg_resources directly for the same
info.

Related issues:

pypa/pip#5079
pypa/pip#5080
pypa/pip#5081

Author: Chris Schneider

src/scout_apm/core/__init__.py

@@ -1,13 +1,5 @@
 from __future__ import absolute_import
 
-
-# pip needs to be imported before anything else (in particular the requests
-# library), since it vendors stuff oddly.
-#
-# This is an unsupported use of pip, and we need to figure out a smoother way
-# to detect loaded libraries. Check metadata.py for our usage.
-import pip # noqa
-
 # Python Modules
 import logging
 from os import getpid

src/scout_apm/core/metadata.py

@@ -4,7 +4,6 @@
 from datetime import datetime
 import logging
 from os import getpid
-import pip
 import sys
 
 
@@ -44,7 +43,7 @@ def data(cls):
                     'database_engine':    '',  # Detected
                     'database_adapter':   '',  # Raw
                     'application_name':   '',  # Environment.application_name,
-                    'libraries':          cls.package_list(),
+                    'libraries':          cls.get_python_packages_versions(),
                     'paas':               '',
                     'git_sha':            ''}  # Environment.git_revision_sha()}
         except Exception as e:
@@ -53,16 +52,13 @@ def data(cls):
         return data
 
     @classmethod
-    def package_list(cls):
-        packages = []
+    def get_python_packages_versions(cls):
         try:
-            for pkg_dist in [p for p in pip._vendor.pkg_resources.working_set]:
-                try:
-                    p_split = str(pkg_dist).split()
-                    packages.append([p_split[0], p_split[1]])
-                except Exception as e:
-                    logger.debug('Exception while reading packages: %s', repr(e))
-                    continue
-        except Exception as e:
-            logger.debug('Exception while getting pkg_resources: %s', repr(e))
-        return packages
+            import pkg_resources
+        except ImportError:
+            return []
+
+        return list(sorted(
+            (distribution.project_name, distribution.version)
+            for distribution in pkg_resources.working_set
+        ))