Refactor for xplat building. (#140)

Compare to
#139. I propose
this is more correct for how we want to make this xplat.

Verified this builds and runs on Mac and Windows, and Testing passes on
both.

Author: jeffdav

Package.swift

@@ -5,28 +5,37 @@ import PackageDescription
 let package = Package(
     name: "swift-webdriver",
     products: [
-        .library(name: "WebDriver", targets: ["WebDriver", "WinAppDriver"]),
+        .library(name: "WebDriver", targets: ["WebDriver"]),
     ],
     targets: [
         .target(
             name: "WebDriver",
             path: "Sources/WebDriver"),
-        .target(
-            name: "WinAppDriver",
-            dependencies: ["WebDriver"],
-            path: "Sources/WinAppDriver"),
         .target(
             name: "TestsCommon",
             path: "Tests/Common"),
-        .testTarget(
-            name: "WinAppDriverTests",
-            dependencies: ["TestsCommon", "WebDriver", "WinAppDriver"],
-            // Ignore "LNK4217: locally defined symbol imported" spew due to SPM library support limitations
-            linkerSettings: [ .unsafeFlags(["-Xlinker", "-ignore:4217"]) ]),
         .testTarget(
             name: "UnitTests",
-            dependencies: ["TestsCommon", "WebDriver", "WinAppDriver"],
+            dependencies: ["TestsCommon", "WebDriver"],
             // Ignore "LNK4217: locally defined symbol imported" spew due to SPM library support limitations
-            linkerSettings: [ .unsafeFlags(["-Xlinker", "-ignore:4217"]) ]),
+            linkerSettings: [ .unsafeFlags(["-Xlinker", "-ignore:4217"], .when(platforms: [.windows])) ]),
     ]
 )
+
+#if os(Windows)
+package.products += [
+    .library(name: "WinAppDriver", targets: ["WinAppDriver"])
+]
+package.targets += [
+    .target(
+        name: "WinAppDriver",
+        dependencies: ["WebDriver"],
+        path: "Sources/WinAppDriver"),
+    .testTarget(
+        name: "WinAppDriverTests",
+        dependencies: ["TestsCommon", "WebDriver", "WinAppDriver"],
+        // Ignore "LNK4217: locally defined symbol imported" spew due to SPM library support limitations
+        linkerSettings: [ .unsafeFlags(["-Xlinker", "-ignore:4217"]) ]),
+]
+#endif
+

Sources/WebDriver/HTTPWebDriver.swift

@@ -1,5 +1,7 @@
 import Foundation
+#if canImport(FoundationNetworking)
 import FoundationNetworking
+#endif
 
 public struct HTTPWebDriver: WebDriver {
     let rootURL: URL

Sources/WebDriver/URLRequestExtensions.swift

@@ -1,5 +1,7 @@
 import Foundation
+#if canImport(FoundationNetworking)
 import FoundationNetworking
+#endif
 
 extension URLSession {
     func dataTask(