jbuilder exec doesn't work in sub directories

[rgrinberg]

When using relative paths. E.g. jbuilder exec ./main.exe. E.g. trying this in cohttp, I get:

File "jbuild", line 20, characters 11-17:
Error: You cannot declare items to be installed without adding a <package>.opam file at the root of your project.
To declare elements to be installed as part of package "cohttp", add a "cohttp.opam" file at the root of your project.

cc @bobot