`dist/` should not be deleted on rebuilds, only contents instead

@jvandemo heya, I recently discovered a [shortcoming in my starter](https://github.com/filipesilva/angular-quickstart-lib/commit/ce13ea3ee5a962e5c46edeef7678e8339265a74d), where the `dist/` dir is always deleted.

This causes linked libraries to not update the project that are using it, since the link is broken when `dist/` is deleted. Instead, only its contents should be deleted.

Details and repro can be found at https://github.com/angular/angular-cli/issues/6195.

Start by reproducing the linked-library problem described in the issue and inspect the rebuild step that removes dist/. Change the behavior so the dist/ directory remains while its contents are cleared, then verify that linked libraries continue updating the consuming project. Use the Angular CLI issue 6195 for the detailed reproduction.

Written by the indexing model from the issue text.