Allow dependencies to be specified explicitely rather than through `-I`.

For instance in odig I already have the precise info, I prune it into -I and odoc does the lookup again which is absurd.

Note that it may be better to allow to specify them in a file (e.g. one per line) rather than on the cli proper since IIRC some environments have low limits on the number of cli arguments you can specify.

Ideally the scheme would be the same as the one which would be chosen for the compiler (e.g. see this). I guess @lpw25 must have something in mind for that in the context of his namespace proposal.

Start with odoc's -I dependency lookup and compare it with odig's existing pruned dependency information. Review the linked compiler scheme and namespace proposal before deciding whether explicit dependencies should be passed on the CLI or through a file. Done means odoc can consume explicit dependencies without repeating the lookup and follows the compiler's chosen scheme.