Use virtual_modules in Dune 1.7

[avsm]

This allows use of the interface file separately from the implementations. We now provide separate implementations that use dune variants so that they can be found via a variants tag search:

• unix (Unix or Windows)
• js (JavaScript)
• xen (Xen)

This cleans up the utop toplevel support as well to work by default now. (@avsm, fixes #50)
Also use implicit transitive dependencies, which will become the default in dune 2.0

I am noticing some local problems with opam installations though:

#=== ERROR while compiling io-page-unix.2.2.0 =================================#
# context     2.0.4 | macos/x86_64 | ocaml-base-compiler.4.07.1 | pinned(git+file:///Users/avsm/src/git/mirage/io-page#vmodules#a58f53d0)
# path        ~/.opam/ocaml-base-compiler.4.07.1/.opam-switch/build/io-page-unix.2.2.0
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p io-page-unix -j 27
# exit-code   1
# env-file    ~/.opam/log/io-page-unix-37329-03b1a4.env
# output-file ~/.opam/log/io-page-unix-37329-03b1a4.out
### output ###
# File "lib/unix/dune", line 7, characters 14-21:
# 7 |   (implements io-page)
#                   ^^^^^^^
# Error: Library implementation io-page for variant "unix" implements a library outside the project. This is forbidden.

which is why I am marking this as a draft PR to work through the CI first.

[djs55]

awesome!

[dinosaure]

Should related to mirage/irmin#338

[hannesm]

I still don't fully understand variants, but in the above list, what is happening with freestanding? or is this a non-issue? AFAIR the C stubs are provided by mirage-solo5 atm, but what does dune do when asked to use a variant that does not exist for a certain package? take the "default" one (is there a default?)?

and what is the extensibility story - do all variants now need to end up in this io-page repository, or are they free to be defined elsewhere? (how will people/programs know about the existance elsewhere, what happens if there are multiple implementations of a single package for the same variant?)

thanks for working on this! and sorry for asking questions, but I could not discover where/whether they've been answered previously (please point me to a comment if this is the case)

[samoht]

Do we really need variants when we could just rely on dune doing the correct cross-compilation of C bindings? e.g. in #60 I've simply removed the unix and xen subpackages and everything seems much cleaner/simpler :-)

[samoht]

Closing in favour of #60