When bootstrapping SwiftPM, use relative paths for dependencies.

When bootstrapping SwiftPM, SwiftPM uses relative paths to find
its dependencies (e.g., llbuild) rather than repository URLs.
Do the same thing for the driver when SWIFTCI_USE_LOCAL_DEPS is set in
the environment. This allows us to use the Swift driver when
bootstrapping SwiftPM.

Author: DougGregor

Package.swift

@@ -1,5 +1,6 @@
 // swift-tools-version:5.1
 import PackageDescription
+import class Foundation.ProcessInfo
 
 let package = Package(
   name: "swift-driver",
@@ -20,11 +21,6 @@ let package = Package(
       name: "SwiftOptions",
       targets: ["SwiftOptions"]),
   ],
-  dependencies: [
-    .package(url: "https://github.com/apple/swift-tools-support-core.git", .branch("master")),
-    .package(url: "https://github.com/apple/swift-llbuild.git", .branch("master")),
-    .package(url: "https://github.com/jpsim/Yams.git", .branch("master")),
-  ],
   targets: [
     /// The driver library.
     .target(
@@ -59,3 +55,17 @@ let package = Package(
   ],
   cxxLanguageStandard: .cxx14
 )
+
+if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
+  package.dependencies += [
+    .package(url: "https://github.com/apple/swift-tools-support-core.git", .branch("master")),
+    .package(url: "https://github.com/apple/swift-llbuild.git", .branch("master")),
+    .package(url: "https://github.com/jpsim/Yams.git", .branch("master")),
+    ]
+} else {
+    package.dependencies += [
+        .package(path: "../swift-tools-support-core"),
+        .package(path: "../yams"),
+        .package(path: "../llbuild"),
+    ]
+}