Eliminate read overhead for in-source layouts

What happens when the `directory` option is set inside the `metalsmith.source()`, eg.

```js
metalsmith
.source('src')
.use(layouts({
directory: 'src/layouts'
}))
// currently requires @metalsmith/remove?
.use(remove('layouts'))
```

There is a limited set of cases where removing the layouts may not be desirable (eg. a design system that needs to show the source templates)

Resolution of this issue:
- Add a test case
- Add an option that defaults to current behavior for non-semver breaking release

Start by locating the layouts plugin's handling of the directory option and its existing test suite. Add coverage for a source directory containing layouts, then verify that the new option controls removal while the default preserves current behavior.

Written by the indexing model from the issue text.