Support '--use-feature' in requirements files

This patch adds support for `--use-feature` in requirements files
so that a project that wants all contributors using the same pip
features can specify it in the requirements file. For example, to ensure
a requirements file uses the new resolver:

```
--use-feature=2020-resolver
boto3
boto3==1.13.13
```

This is a new version of pypa#8293.

news/8601.feature

@@ -0,0 +1 @@
+Support ``--use-feature`` in requirements files

src/pip/_internal/req/req_file.py

@@ -62,6 +62,7 @@
     cmdoptions.require_hashes,
     cmdoptions.pre,
     cmdoptions.trusted_host,
+    cmdoptions.use_new_feature,
 ]  # type: List[Callable[..., optparse.Option]]
 
 # options to be passed to requirements

tests/unit/test_req_file.py

@@ -382,6 +382,10 @@ def test_set_finder_allow_all_prereleases(self, line_processor, finder):
         line_processor("--pre", "file", 1, finder=finder)
         assert finder.allow_all_prereleases
 
+    def test_use_feature(self, line_processor):
+        """--use-feature can be set in requirements files."""
+        line_processor("--use-feature=2020-resolver", "filename", 1)
+
     def test_relative_local_find_links(
         self, line_processor, finder, monkeypatch, tmpdir
     ):