Consider adding a flag to specify alternate sfdx-project.json files

[DanielAEKalil]

There are scenarios in which we want to have a different set of packageDirectories in different contexts; primarily, during the development of a major feature-branch.

It would be really great if we could provide a path value via a flag like --project-json and, if specified, the getSfdxProjectJson() in src/utils/project.js would use that path value (but would otherwise still default to using the cwd).