Default to 'optional' directory when the providing modules install config

Problem/Motivation

Many modules install default config. If we enforce this config, and put it into the 'install' directory of our target module, installing the target module will fail; complaining that the config already exists in active config.

Proposed resolution

When building the enforced config form, we could try to determine if that config already exists in any module's `config/install` directory. If yes, we could default to storing the enforced config in `config/optional`, and perhaps even disabling the `config/install` option.

Remaining tasks

TBD