PackageLoading: escape \ when generating the modulemap

The `pathString` is overloaded and used as both a printing string and a
path string.  When used to generate the module map, we need to escape
the path separator as it is an escape sequence indicator otherwise.

This allows us to generate modulemaps which can be consumed by clang
on Windows.

Author: compnerd

Sources/PackageLoading/ModuleMapGenerator.swift

@@ -10,9 +10,16 @@
 
 import TSCBasic
 import PackageModel
+import Foundation
 
 public let moduleMapFilename = "module.modulemap"
 
+extension AbsolutePath {
+  fileprivate var moduleEscapedPathString: String {
+    return self.pathString.replacingOccurrences(of: "\\", with: "\\\\")
+  }
+}
+
 /// A protocol for targets which might have a modulemap.
 protocol ModuleMapProtocol {
     var moduleMapPath: AbsolutePath { get }
@@ -152,9 +159,9 @@ public struct ModuleMapGenerator {
         stream <<< "module \(target.c99name) {\n"
         switch type {
         case .header(let header):
-            stream <<< "    umbrella header \"\(header.pathString)\"\n"
+            stream <<< "    umbrella header \"\(header.moduleEscapedPathString)\"\n"
         case .directory(let path):
-            stream <<< "    umbrella \"\(path.pathString)\"\n"
+            stream <<< "    umbrella \"\(path.moduleEscapedPathString)\"\n"
         }
         stream <<< "    export *\n"
         stream <<< "}\n"