duplicates:

• SR-4400 Resolver is doing too much work

relates to:

• SR-4400 Resolver is doing too much work

Issue Description:

SPM 3.1 spins when fetching dependencies with invalid tags.
The following Package.swift source will try and depend on several packages which have non-existent tags (0,0,0). Calling `swift package fetch` will seemingly spin forever.

I did not see the problem with only one bad dependency. In that case I got the right answer of "error: unsatisfiable"

// swift-tools-version:3.1

import PackageDescription

let package = Package(
name: "TestProj",
targets: [],
dependencies: [
.Package(url: "https://github.com/PerfectlySoft/Perfect-FileMaker.git", majorVersion: 2),
.Package(url: "https://github.com/SwiftORM/SQLite-StORM.git", Version(0,0,0)),
.Package(url: "https://github.com/SwiftORM/Postgres-StORM.git", Version(0,0,0)),
.Package(url: "https://github.com/SwiftORM/MongoDB-StORM.git", Version(0,0,0)),
.Package(url: "https://github.com/SwiftORM/MySQL-StORM.git", Version(0,0,0)),
.Package(url: "https://github.com/PerfectlySoft/Perfect-HTTPServer.git", majorVersion: 2),
.Package(url: "https://github.com/PerfectlySoft/Perfect-WebSockets.git", majorVersion: 2),
.Package(url: "https://github.com/PerfectlySoft/Perfect-Notifications.git", majorVersion: 2),
.Package(url: "https://github.com/PerfectlySoft/Perfect-Zip.git", majorVersion: 2),
.Package(url: "https://github.com/PerfectlySoft/Perfect-Mustache.git", majorVersion: 2),
.Package(url: "https://github.com/PerfectlySoft/Perfect-CURL.git", majorVersion: 2),

],
exclude: []
)