New opam field to specify the location of the .install file

[dbuenzli]

Currently a .install file needs to be written at the root of the package build directory to be considered.

However the natural location of such a file is rather in the build directory of the build system: this make it easy to clean and need no special line in our .gitignores.

So I think it would be nice to have a field

install-file: <string>

that allows to specify its location w.r.t. to the root of the package build directory (the semantics of the .install file remains however exactly the same, i.e. relative file paths therein are expressed relative to the root of the package build directory).

If something like #3797 becomes a reality end users won't be even bothered to have to write this down as most opam files will end-up being generated anyways.