RFE: local plugins and build-tool-depends

This is a very ambituous request because it goes beyond #2965

I am the author of a compiler plugin and command line tool (both at https://gitlab.com/tseenshe/hsinspect) that are only useful for developers and not something anybody would ever want to export in their `.cabal` file.

I know of other plugins that are only useful for developers and not for the build itself, e.g. the removal of unused imports.

I would like for users to be able to install my plugin and command line tool via their `cabal.project.local` file.