Implement the "conflicts:" field.
Retrieving OPAM package: when warning, a default is used.

When warning the user that a package for a Findlib library was not
found, one uses the findlib name as the default (packages providing a
single library often bear the same name).
Output the opam file using the Format module.
Determine better Findlib libraries to be installed.
Keep the set of versions for OPAM packages providing a given library.

The motivation is that, for example, conflicts may depend on the
package version.  If a package is changed to remove a conflict, the
later versions should not be present in the "conflicts" field.
Move the decision about compulsory/optional dependencies.