duplicates:

• SR-4072 Better response rather than "unsatisfiable"

relates to:

• SR-4124 Dependency resolution error for unsatisfiable dependencies due to tools version should say why
• SR-4072 Better response rather than "unsatisfiable"

Issue Description:

It's easy to get unsatisfiable dependencies due to pinning and not realize that pinning is the problem. The error that SwiftPM gives when you have unsatisfiable dependencies should mention that dependency restrictions are coming from your pinfile.

Here's an example workflow where this is very confusing:

1. Clone vapor git@github.com:vapor/vapor.git
2. Check out tag 2.0.0-alpha.7
3. Run `swift build`
4. Check out tag 1.5.9
5. Run `swift build`

Result: SwiftPM emits "error: unsatisfiable". The problem here is that your earlier build left you with a Package.pins file that specifies dependencies that are incompatible with 1.5.9. In order to build you need to `rm Package.pins`, but that's really non-obvious.
The error message should in some way indicate that pinning is part of the dependency resolution that's unsatisfiable.

This is related to / a potential subtask of [SR-4237 Unsatisfiable dependency error should say which dependency can't be satisfied](https://bugs.swift.org/browse/SR-4237)