This is only a transitional measure that can be temporarily used by
users to help them to adapt to the PEP 660 implementation.

In this commit the argument for the `editable_wheel` command is changed
from the flag form `--strict` to `--mode=strict`.

The `exec` function in Python should be able to execute code directly.
Using `compile` and then `exec` seem to cause an overhead.

PEP 517 does not care if other directories/files are left behind in the
`metadata_directory`, as long as a `.dist_info` directory is produced at
the root. We can leave the `.egg-info` directory behind, so this way
we don't have to run it again when listing files from `build_py`.