Remove vendored pip-diff script (#1118)

Since the last usage was removed in #1117.

Whilst this tool was intended for internal buildpack usage only, the
vendor directory is on `PATH`, so I've documented the removal in
CHANGELOG just in case.

Refs @W-8208817@.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Remove vendored `pip-diff` script (#1118).
 - Remove vendored `pip-grep` script (#1116).
 
 ## v185 (2020-11-12)

vendor/pip-pop/pip-diff

@@ -1,132 +0,0 @@
-#!/usr/bin/env python
-# -*- coding: utf-8 -*-
-
-"""Usage:
-  pip-diff (--fresh | --stale) <reqfile1> <reqfile2> [--exclude <package>...]
-  pip-diff (-h | --help)
-
-Options:
-  -h --help     Show this screen.
-  --fresh       List newly added packages.
-  --stale       List removed packages.
-"""
-import os
-from docopt import docopt
-
-try:  # pip >= 10
-    from pip._internal.req import parse_requirements
-    from pip._internal.download import PipSession as session
-
-    def PackageFinder(find_links, index_urls, session=None):
-        from pip._internal.index import PackageFinder
-        from pip._internal.models.search_scope import SearchScope
-        from pip._internal.models.selection_prefs import SelectionPreferences
-
-        search_scope = SearchScope.create(find_links, index_urls)
-        selection_prefs = SelectionPreferences(allow_yanked=False)
-        return PackageFinder.create(search_scope, selection_prefs, session=session)
-
-except ImportError:  # pip <= 9.0.3
-    from pip.req import parse_requirements
-    from pip.index import PackageFinder
-    from pip._vendor.requests import session
-
-requests = session()
-
-
-class Requirements(object):
-    def __init__(self, reqfile=None):
-        super(Requirements, self).__init__()
-        self.path = reqfile
-        self.requirements = []
-
-        if reqfile:
-            self.load(reqfile)
-
-    def __repr__(self):
-        return '<Requirements \'{}\'>'.format(self.path)
-
-    def load(self, reqfile):
-        if not os.path.exists(reqfile):
-            raise ValueError('The given requirements file does not exist.')
-
-        finder = PackageFinder([], [], session=requests)
-        for requirement in parse_requirements(reqfile, finder=finder, session=requests):
-            if requirement.req:
-                if not getattr(requirement.req, 'name', None):
-                    # Prior to pip 8.1.2 the attribute `name` did not exist.
-                    requirement.req.name = requirement.req.project_name
-                    requirement.req.name = requirement.req.name.lower()
-                self.requirements.append(requirement.req)
-
-
-    def diff(self, requirements, ignore_versions=False, excludes=None):
-        r1 = self
-        r2 = requirements
-        results = {'fresh': [], 'stale': []}
-
-        # Generate fresh packages.
-        other_reqs = (
-            [r.name for r in r1.requirements]
-            if ignore_versions else r1.requirements
-        )
-
-        for req in r2.requirements:
-            r = req.name if ignore_versions else req
-
-            if r not in other_reqs and r not in excludes:
-                results['fresh'].append(req)
-
-        # Generate stale packages.
-        other_reqs = (
-            [r.name for r in r2.requirements]
-            if ignore_versions else r2.requirements
-        )
-
-        for req in r1.requirements:
-            r = req.name if ignore_versions else req
-
-            if r not in other_reqs and r not in excludes:
-                results['stale'].append(req)
-
-        return results
-
-
-def diff(r1, r2, include_fresh=False, include_stale=False, excludes=None):
-    include_versions = True if include_stale else False
-    excludes = excludes if len(excludes) else []
-
-    try:
-        r1 = Requirements(r1)
-        r2 = Requirements(r2)
-    except ValueError:
-        print('There was a problem loading the given requirements files.')
-        exit(os.EX_NOINPUT)
-
-    results = r1.diff(r2, ignore_versions=True, excludes=excludes)
-
-    if include_fresh:
-        for line in results['fresh']:
-            print(line.name if include_versions else line)
-
-    if include_stale:
-        for line in results['stale']:
-            print(line.name if include_versions else line)
-
-
-def main():
-    args = docopt(__doc__, version='pip-diff')
-
-    kwargs = {
-        'r1': args['<reqfile1>'],
-        'r2': args['<reqfile2>'],
-        'include_fresh': args['--fresh'],
-        'include_stale': args['--stale'],
-        'excludes': args['<package>']
-    }
-
-    diff(**kwargs)
-
-
-if __name__ == '__main__':
-    main()