Fix for explicit module build

Use the Swift modf rather than the C modf and add a missing direct
dependency.

Author: hjyamauchi

Package.swift

@@ -208,6 +208,7 @@ let SwiftFirebase =
                       ]),
               .executableTarget(name: "FireBaseUI",
                                 dependencies: [
+                                  "firebase",
                                   "FirebaseCore",
                                   "FirebaseAuth",
                                   "FirebaseFirestore",
@@ -219,6 +220,7 @@ let SwiftFirebase =
                                 cxxSettings: [
                                   .define("INTERNAL_EXPERIMENTAL"),
                                   .define("_CRT_SECURE_NO_WARNINGS", .when(platforms: [.windows])),
+                                  .headerSearchPath("../../third_party/firebase-development/usr/include"),
                                 ],
                                 swiftSettings: [
                                   .interoperabilityMode(.Cxx),

Sources/FirebaseFirestore/Timestamp+Swift.swift

@@ -49,8 +49,7 @@ extension Timestamp: Codable {
 
 extension Date {
   internal func firestoreTimestamp() -> Timestamp {
-    var secondsDouble: Double = 0.0
-    var fraction = modf(timeIntervalSince1970, &secondsDouble)
+    var (secondsDouble, fraction) = modf(timeIntervalSince1970)
 
     // Re-implementation of https://github.com/firebase/firebase-ios-sdk/blob/master/Firestore/Source/API/FIRTimestamp.m#L50
     if (fraction < 0) {