Add integration tests for experimental feature fast-deps

Test pip {download,install,wheel} where requirements have dependencies
listed in their wheels' metadata.

tests/data/packages/requiresPaste/pyproject.toml

@@ -0,0 +1,9 @@
+[build-system]
+requires = ['flit_core >=2,<4']
+build-backend = 'flit_core.buildapi'
+
+[tool.flit.metadata]
+module = 'requiresPaste'
+author = 'A. Random Developer'
+author-email = 'author@example.com'
+requires = ['Paste==3.4.2']

tests/data/packages/requiresPaste/requiresPaste.py

@@ -0,0 +1,3 @@
+"""Module requiring Paste to test dependencies download of pip wheel."""
+
+__version__ = '3.1.4'

tests/functional/test_fast_deps.py

@@ -0,0 +1,50 @@
+import fnmatch
+import json
+from os.path import basename
+
+from pip._vendor.packaging.utils import canonicalize_name
+from pytest import mark
+
+
+def pip(script, command, requirement):
+    return script.pip(
+        command, '--prefer-binary', '--no-cache-dir',
+        '--use-feature=fast-deps', requirement,
+        allow_stderr_warning=True,
+    )
+
+
+def assert_installed(script, names):
+    list_output = json.loads(script.pip('list', '--format=json').stdout)
+    installed = {canonicalize_name(item['name']) for item in list_output}
+    assert installed.issuperset(map(canonicalize_name, names))
+
+
+@mark.network
+@mark.parametrize(('requirement', 'expected'), (
+    ('Paste==3.4.2', ('Paste', 'six')),
+    ('Paste[flup]==3.4.2', ('Paste', 'six', 'flup')),
+))
+def test_install_from_pypi(requirement, expected, script):
+    pip(script, 'install', requirement)
+    assert_installed(script, expected)
+
+
+@mark.network
+@mark.parametrize(('requirement', 'expected'), (
+    ('Paste==3.4.2', ('Paste-3.4.2-*.whl', 'six-*.whl')),
+    ('Paste[flup]==3.4.2', ('Paste-3.4.2-*.whl', 'six-*.whl', 'flup-*')),
+))
+def test_download_from_pypi(requirement, expected, script):
+    result = pip(script, 'download', requirement)
+    created = list(map(basename, result.files_created))
+    assert all(fnmatch.filter(created, f) for f in expected)
+
+
+@mark.network
+def test_build_wheel_with_deps(data, script):
+    result = pip(script, 'wheel', data.packages/'requiresPaste')
+    created = list(map(basename, result.files_created))
+    assert fnmatch.filter(created, 'requiresPaste-3.1.4-*.whl')
+    assert fnmatch.filter(created, 'Paste-3.4.2-*.whl')
+    assert fnmatch.filter(created, 'six-*.whl')