[PackageLoading] Lift no modules error up a level.

 - There is nothing malformed about a package with no modules, it just happens
   we have a policy that makes it an error for non-root packages. That is better
   enforced at a level above the package loading.

Author: ddunbar

Sources/PackageGraph/PackageGraphLoader.swift

@@ -17,21 +17,29 @@ import PackageLoading
 import func POSIX.exit
 
 enum PackageGraphError: Swift.Error {
+    /// Indicates two modules with the same name.
     case duplicateModule(String)
+
+    /// Indicates a non-root package with no modules.
+    case noModules(Package)
 }
 
 extension PackageGraphError: FixableError {
     var error: String {
         switch self {
         case .duplicateModule(let name):
             return "multiple modules with the name \(name) found"
+        case .noModules(let package):
+            return "the package \(package) contains no modules"
         }
     }
 
     var fix: String? {
         switch self {
         case .duplicateModule(_):
             return "modules should have a unique name across dependencies"
+        case .noModules(_):
+            return "create at least one module"
         }
     }
 }
@@ -70,14 +78,21 @@ public struct PackageGraphLoader {
             let isRootPackage = (i + 1) == allManifests.count
             packages.append(package)
 
-            var modules: [Module]
-            do {
-                modules = try package.modules()
-            } catch ModuleError.noModules(let pkg) where isRootPackage {
-                // Ignore and print warning if root package doesn't contain any sources.
-                print("warning: root package '\(pkg)' does not contain any sources")
-                if allManifests.count == 1 { exit(0) } //Exit now if there is no more packages 
-                modules = []
+            let modules: [Module] = try package.modules()
+
+            // Diagnose empty non-root packages, which are something we allow as a special case.
+            if modules.isEmpty {
+                if isRootPackage {
+                    // Ignore and print warning if root package doesn't contain any sources.
+                    print("warning: root package '\(package)' does not contain any sources")
+                    
+                    // Exit now if there are no more packages.
+                    //
+                    // FIXME: This does not belong here.
+                    if allManifests.count == 1 { exit(0) }
+                } else {
+                    throw PackageGraphError.noModules(package)
+                }
             }
 
             // TODO: Allow testing of external package tests.

Sources/PackageLoading/PackageExtensions.swift

@@ -15,7 +15,6 @@ import Utility
 import class PackageDescription.Target
 
 public enum ModuleError: Swift.Error {
-    case noModules(Package)
     case modulesNotFound([String])
     case invalidLayout(InvalidLayoutType)
     case executableAsDependency(module: String, dependency: String)
@@ -29,8 +28,6 @@ public enum InvalidLayoutType {
 extension ModuleError: FixableError {
     public var error: String {
         switch self {
-        case .noModules(let package):
-            return "the package \(package) contains no modules"
         case .modulesNotFound(let modules):
             return "these referenced modules could not be found: " + modules.joined(separator: ", ")
         case .invalidLayout(let type):
@@ -42,8 +39,6 @@ extension ModuleError: FixableError {
 
     public var fix: String? {
         switch self {
-        case .noModules(_):
-            return "create at least one module"
         case .modulesNotFound(_):
             return "reference only valid modules"
         case .invalidLayout(let type):
@@ -186,10 +181,12 @@ extension Package {
 
         let modules: [Module]
         if maybeModules.isEmpty {
+            // If there are no sources subdirectories, we have at most a one target package.
             do {
                 modules = [try modulify(srcroot, name: self.name, isTest: false)]
             } catch Module.Error.noSources {
-                throw ModuleError.noModules(self)
+                // Completely empty packages are allowed as a special case.
+                modules = []
             }
         } else {
             modules = try maybeModules.map { path in