Issue Description:

When running swift package resolve with the following manifest, it gets stuck in an infinite loop. The reason is that fluent-postgresql is only a major version 1, 3.0.0 doesn't exist yet.

Steps to reproduce:

1. Create empty SPM project

2. Set Package.swift to the following:
   // swift-tools-version:4.2
import PackageDescription``let package = Package(
   {{ name: "name",}}
   {{ dependencies: [}}
   {{ .package(url: "https://github.com/vapor/vapor.git", from: "3.0.0"),}}
   {{ .package(url: "https://github.com/vapor/fluent-postgresql.git", from: "3.0.0"),}}
   {{ .package(url: "https://github.com/vapor/auth.git", from: "2.0.0"),}}
   {{ ],}}
   {{ targets: [}}
   {{ .target(name: "App", dependencies: ["Authentication", "FluentPostgreSQL", "Vapor"]),}}
   {{ .target(name: "Run", dependencies: ["App"]),}}
   {{ .testTarget(name: "AppTests", dependencies: ["App"])}}
   {{ ]}}
   )

3. Run swift package resolve

Expected outcome:

• Error produced showing could not be satisfied

• Ideally says 3.0.0 does not exist

Actual outcome:
SPM gets stuck in an infinite loop