list opam packages

[samoht]

If thing that I often do when trying to debug a failing build is to check the version of the installed packages. With travis, this means scrolling a bit to find opam actions.

With obi it's not possible to scroll as the logs are trimed a bit; instead it would be nice to have a way to list either the packages which were planned to be installed and/or the list of packages successfully installed without the error.

[avsm]

We have the right metadata available for this in the Builds branch, since the raw OpamJson is saved. To implement this, we need to:

• add support in src/lib/opamJsonActions.ml for extracting the installed packages (right now we just use it to determine what was built/skipped)
• store this information in src/lib/obi.ml in the Index metadata so that it can be accessed by opam-ci.

It will increase the size of the metadata, but we need to do this per-build and not per-package since each build-time configuration could select different packages due to differing compiler/os/arch constraints. If size becomes a problem, we can compress the index.sxp metadata.

[kit-ty-kate]

Slightly related to #22