Usability issues with the way dune handles headers

I've looked over @aantron's luv library, and I noticed that there are too many contortions required to use C headers the way one wants. Even with all the contortions, one still does not get a desirable result. Here's a list of the issues:

• To preserve the file structure of the headers (as is often required), one is required to write install stanzas manually like here https://github.com/aantron/luv/blob/44db25c2b2669f9d1373377006a4d5850e0820e3/src/c/dune#L15

• Introducing a dependency on the library to which the headers belong does not introduce a dependency on the headers. To depend on the headers, one has to add a (package luv) dependency somewhere.

• When the package containing the headers is installed, things are still broken because dependencies aren't inferred correctly. Dune only adds dependencies on the header files in the root of the package, and omits dependencies on headers in sub directories.

Start with the linked luv file, src/c/dune, and trace how Dune handles C-header installation and dependency inference. Done should address the listed cases: preserve header structure without manual install stanzas, infer header dependencies from library dependencies, and include headers in subdirectories.