Assume built rewriting

[rjbou]

Some fixes:

• handling of post dependencies (leading to an infinite look up of dependencies, or dependency unavailable)
• specific missing dep message

[lthls]

I've tried this PR, using the instructions on the wiki here, with the compiler.

The setup part, in addition the setting opam2 to use the experimental version with a distinguished root, includes checking out a compiler branch I'm interesting in testing and cding into it.
At this point the current directory is a fresh copy, not configured.

Then I ran the following commands:

$ opam2 sw cr test --empty
$ opam2 pin add . --inplace-build # I think using install would have worked too
# At this point, I start making changes to the compiler, recompiling, testing using the compiler's testsuite
# Then when I'm ready to test on opam:
$ make -j4 world.opt # just in case
$ opam install . --inplace-build --assume-built

The last command successfully ran, installing the compiler without recompiling it from scratch.

The main pain point is that the first pin command has to be run to configure the compiler correctly, but I didn't find an easy way to tell it to stop after configure so it has to build everything (Although I could have interrupted the process with Ctrl+C, I think).
Otherwise, this looks quite useful (especially on flambda2, where compiling the compiler can take quite long).