Dependency version with invalid direct dependencies should be tracked as incompatibility

[zanieb]

Example at https://github.com/zanieb/pubgrub/blob/zanie/examples/examples/unsat_indirect_dependency_conflict_backtrack.rs

Given the following situation:

• root depends on foo
• foo 1.1 depends on bar 1.0
• foo 1.2 depends on bar 1.0 and bar 2.0 (invalid)

The solver fails with an empty dependency set on foo 1.2.
Instead, the solver should mark foo 1.2 as incompatible(?) and backtrack to a valid version of foo such as 1.1.

[zanieb]

In general, we need to be able to indicate that some package version does not have valid dependencies and should be ignored with a custom message for reports. I opened #152 to track that.

[Eh2406]

The solver fails with an empty dependency set on foo 1.2.

Is this the DependencyOnTheEmptySet error? There is discussion about removing it in #125 and it was removed in #133, please try the dev branch and let me know if it's still a problem.

[zanieb]

Ah thank you — I misunderstood the main branch in this project and have rebased my examples onto the dev branch.

This example indeed solves with dev.