Use pkg_resources instead of pip for is_package (resolves #1293)

requirements.txt

@@ -13,7 +13,6 @@ requests>=2.13.0,<3.0.0
 regex==2017.4.5
 ftfy>=4.4.2,<5.0.0
 pytest>=3.0.6,<4.0.0
-pip>=9.0.0,<10.0.0
 mock>=2.0.0,<3.0.0
 msgpack-python
 msgpack-numpy

setup.py

@@ -197,7 +197,6 @@ def setup_package():
                 'preshed>=1.0.0,<2.0.0',
                 'thinc>=6.8.1,<6.9.0',
                 'plac<1.0.0,>=0.9.6',
-                'pip>=9.0.0,<10.0.0',
                 'six',
                 'pathlib',
                 'ujson>=1.35',

spacy/util.py

@@ -3,7 +3,7 @@ from __future__ import unicode_literals, print_function
 
 import os
 import ujson
-import pip
+import pkg_resources
 import importlib
 import regex as re
 from pathlib import Path
@@ -180,9 +180,9 @@ def is_package(name):
     name (unicode): Name of package.
     RETURNS (bool): True if installed package, False if not.
     """
-    packages = pip.get_installed_distributions()
+    packages = pkg_resources.working_set.by_key.keys()
     for package in packages:
-        if package.project_name.replace('-', '_') == name:
+        if package.replace('-', '_') == name:
             return True
     return False