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.

I can't get the functional tests to run locally...

We explicitly propagate --use-feature options from req files upwards.
This is not strictly necessary for the option to be enabled, because
of the default value is a global list, but that implicit behaviour is
certainly accidental, so we make it explicit, with a test.

Percolate --use-feature from req file upwards