Better resolver failure messages

[fredrikekre]

For example from https://stackoverflow.com/questions/52770092/julia-package-dependencies-in-travis-ci/52770252#52770252

ERROR: Unsatisfiable requirements detected for package SchumakerSpline [65e68595]:
     SchumakerSpline [65e68595] log:
     ├─possible versions are: 0.0.1 or uninstalled
     ├─restricted to versions 0.0.1-* by UnivariateFunctions [117ba14f], leaving only versions 0.0.1
     │ └─UnivariateFunctions [117ba14f] log:
     │   ├─possible versions are: 0.0.0 or uninstalled
     │   └─UnivariateFunctions [117ba14f] is fixed to version 0.0.0
     └─restricted by julia compatibility requirements to versions: uninstalled — no versions left

is not very clear IMO. It would be nice to at least print which julia versions the package support etc.

[goerz]

Even though we now show which versions particular packages support, this kind of "tree-view" should be considered internal state, and not shown as part of a user-facing error message. I would recommend keeping it as a @debug log only.

As for a user-facing error message, I think this could boil down to a simple message

Could not resolve the dependency for `PackageName`, available in versions <versions>

followed by two lines in one of the following formats:

• Project <path> has a direct dependency on PackagName <version>
• Package PackageA has a direct dependency on PackageName <version>
• Package PackageB has an indirect dependency on PackageName <version> via <chain of packages>

The <path> would be the path of the project that is being instantiated, and PackageA/PackageB would be a direct dependency in that project.

There might be more conflicts than given by those two lines (which the current tree-view exposes, if you know how to read it), but I think as a user, I only care about the first incompatibility.

This is just an idea (possibly half-baked) that I want to throw out here from the black whole that is Slack

[araujoms]

While @goerz's solution is the ideal one, simply changing the current "restricted to versions <version> by an explicit requirement" to "Project <path> has a direct dependency on PackagName <version>" would already be a massive improvement, and is trivial to implement.

[KristofferC]

Great with some ideas here and I would be happy to see some PRs with example error messages before and after this suggestion.