Revise empty file support for import public

If the file defines no types, but it does have `import public` directives, then
it still needs provide those imports so code only depending on this module gets
the types.

Author: thomasvl

CompileTests/MultiModule/Sources/ReExportAOnly/reexport_a_only.pb.swift

@@ -0,0 +1,16 @@
+// DO NOT EDIT.
+// swift-format-ignore-file
+// swiftlint:disable all
+//
+// Generated by the Swift generator plugin for the protocol buffer compiler.
+// Source: Sources/ReExportAOnly/reexport_a_only.proto
+//
+// For information on using the generated types, please see the documentation:
+//   https://github.com/apple/swift-protobuf/
+
+// Use of 'import public' causes re-exports:
+@_exported import enum ModuleA.E
+@_exported import let ModuleA.Extensions_ext_str
+@_exported import struct ModuleA.A
+
+// This file contained no messages, enums, or extensions.

Protos/CompileTests/MultiModule/Sources/ReExportAOnly/reexport_a_only.proto

@@ -0,0 +1 @@
+import public "Sources/ModuleA/a.proto";

Protos/CompileTests/MultiModule/module_mappings.pbascii

@@ -10,3 +10,7 @@ mapping {
   module_name: "ImportsImportsAPublicly"
   proto_file_path: "Sources/ImportsImportsAPublicly/imports_imports_a_publicly.proto"
 }
+mapping {
+  module_name: "ReExportAOnly"
+  proto_file_path: "Sources/ReExportAOnly/reexport_a_only.proto"
+}

Reference/CompileTests/MultiModule/Sources/ReExportAOnly/reexport_a_only.pb.swift

@@ -0,0 +1,16 @@
+// DO NOT EDIT.
+// swift-format-ignore-file
+// swiftlint:disable all
+//
+// Generated by the Swift generator plugin for the protocol buffer compiler.
+// Source: Sources/ReExportAOnly/reexport_a_only.proto
+//
+// For information on using the generated types, please see the documentation:
+//   https://github.com/apple/swift-protobuf/
+
+// Use of 'import public' causes re-exports:
+@_exported import enum ModuleA.E
+@_exported import let ModuleA.Extensions_ext_str
+@_exported import struct ModuleA.A
+
+// This file contained no messages, enums, or extensions.

Sources/protoc-gen-swift/FileGenerator.swift

@@ -95,31 +95,42 @@ class FileGenerator {
           }
         }
 
-        // If there is nothing to generate, then just record that and be done (usually means
-        // there just was one or more services).
-        let generateEmpty = fileDescriptor.enums.isEmpty && fileDescriptor.messages.isEmpty && fileDescriptor.extensions.isEmpty
-        guard !generateEmpty else {
-            p.print("// This file contained no messages, enums, or extensions.")
-            return
-        }
+        let fileDefinesTypes = !fileDescriptor.enums.isEmpty || !fileDescriptor.messages.isEmpty || !fileDescriptor.extensions.isEmpty
 
+        var hasImports = false
         if fileDescriptor.needsFoundationImport {
             p.print("\(generatorOptions.importDirective.snippet) Foundation")
+          hasImports = true
         }
 
         if fileDescriptor.isBundledProto {
             p.print("// 'import \(namer.swiftProtobufModuleName)' suppressed, this proto file is meant to be bundled in the runtime.")
-        } else {
+            hasImports = true
+        } else if fileDefinesTypes {
             p.print("\(generatorOptions.importDirective.snippet) \(namer.swiftProtobufModuleName)")
+            hasImports = true
         }
 
         let neededImports = fileDescriptor.computeImports(
           namer: namer,
           directive: generatorOptions.importDirective,
           reexportPublicImports: generatorOptions.visibility != .internal)
         if !neededImports.isEmpty {
-            p.print()
+            if hasImports {
+                p.print()
+            }
             p.print(neededImports)
+          hasImports = true
+        }
+
+        // If there is nothing to generate, then just record that and be done (usually means
+        // there just was one or more services).
+        guard fileDefinesTypes else {
+            if hasImports {
+                p.print()
+            }
+            p.print("// This file contained no messages, enums, or extensions.")
+            return
         }
 
         p.print()