FEAT: add a clunky freeze/requirements-like feature to enpkg.

@pberkes This one is for you. This adds the following features to enpkg:

```
# Dump all installed eggs into a requirements.txt-like text file
enpkg --freeze > requirements.txt
# Install this in another environment/installation
enpkg --requirements requirements.txt
```

I have not tested this with multi-layer venv, the exact behaviour we want there is not obvious. I believe the current behaviour is to list all the eggs from every environment in freeze, and then install all of them in the new env with --requirements.

This is not documented in enpkg --help on purpose, as I think the UI is clunky. We can add a similar feature in enpkg2, which would have subcommands.