Fix #89, no more direct calls to ocamlfind. 

See the comment about the drastic performance improvements. 

Note that this suggest that we are not resolving packages (also the current approach won't scale/work when we introduce `pkg-config` support) at the right time. 

We should introduce introduce variable stanzas for each package and package dependencies should resolve to to use of these variables in other variable stanzas rather than ocamlfind calls.  This will reduce the max number of ocamlfind calls to  #package * #phases rather than #components_having_package_as_dep * #package * #phase.