Interface modules by Convention

This discussion spurred a bunch of interest in an alternative way to specify interface files for libraries and (include_subdirs qualifeid). Rather than naming the interface module after the library name or the directory it's inside, use a standard naming convention such as _module_.ml. The advantages are:

• No confusion between Foo and Foo.Foo.
• The ability to rename libraries and module groups without touching dune files or renaming both files and directories
• More familiar to people coming from other languages such as python or rust.

Possible disvantage:

• It could complicate life for merlin.

Start with the linked discussion and the existing (include_subdirs qualified) behavior in Dune. Investigate how interface modules are currently named and how Merlin handles them, then determine the design needed for a convention such as _module_.ml. Done means the convention, its interaction with library and module-group renaming, and Merlin compatibility are specified and implemented.