[PackageBuilder] Emit deterministic diagnostics

Author: aciidgh

Sources/Build/BuildPlan.swift

@@ -18,7 +18,7 @@ import func POSIX.getenv
 public struct BuildParameters {
 
     /// Path to the module cache directory to use for SwiftPM's own tests.
-    fileprivate static let swiftpmTestCache = determineTempDirectory().appending(component: "org.swift.swiftpm.tests")
+    fileprivate static let swiftpmTestCache = determineTempDirectory().appending(component: "org.swift.swiftpm.tests-2")
 
     /// Returns the directory to be used for module cache.
     fileprivate var moduleCache: AbsolutePath {

Sources/PackageLoading/PackageBuilder.swift

@@ -74,7 +74,7 @@ extension ModuleError: FixableError {
         case .overlappingSources(let target, let sources):
             return "The target \(target) has sources overlapping sources: \(sources.map({$0.asString}).joined(separator: ", "))"
         case .multipleLinuxMainFound(let package, let linuxMainFiles):
-            let files = linuxMainFiles.map({ $0.asString }).joined(separator: ", ")
+            let files = linuxMainFiles.map({ $0.asString }).sorted().joined(separator: ", ")
             return "The package \(package) has multiple linux main files: \(files)"
         }
     }