Detect existing package.json

It would be great if our generators could detect an already existing `package.json` file and grab default values from it, and then merge the scaffold changes into the existing file.

We don't want to replace a `package.json` if it already exists -- users may lose things they want in there.

Start by locating the generators that create package.json and the existing package.json handling they use. Verify behavior with an existing package.json containing user values: generator defaults are read, scaffold fields are merged, and existing values are preserved rather than the file being replaced.

Written by the indexing model from the issue text.