[PackageDescription] Add proper resources parameter in PackageDescription

The proposal was accepted so we can remove the underscore now.

<rdar://problem/56589483>

Author: aciidgh

Fixtures/Resources/Simple/Package.swift

@@ -6,14 +6,14 @@ let package = Package(
     targets: [
         .target(
             name: "SwiftyResource",
-            __resources: [
+            resources: [
                 .copy("foo.txt"),
             ]
         ),
 
         .target(
             name: "SeaResource",
-            __resources: [
+            resources: [
                 .copy("foo.txt"),
             ]
         ),

Sources/PackageDescription/Target.swift

@@ -350,8 +350,7 @@ public final class Target {
         path: String? = nil,
         exclude: [String] = [],
         sources: [String]? = nil,
-        // FIXME: Underscored until the evolution process is finished.
-        __resources: [Resource]? = nil,
+        resources: [Resource]? = nil,
         publicHeadersPath: String? = nil,
         cSettings: [CSetting]? = nil,
         cxxSettings: [CXXSetting]? = nil,
@@ -364,7 +363,7 @@ public final class Target {
             path: path,
             exclude: exclude,
             sources: sources,
-            resources: __resources,
+            resources: resources,
             publicHeadersPath: publicHeadersPath,
             type: .regular,
             cSettings: cSettings,

Tests/PackageLoadingTests/PD5LoadingTests.swift

@@ -395,7 +395,7 @@ class PackageDescription5LoadingTests: PackageDescriptionLoadingTests {
                targets: [
                    .target(
                        name: "Foo",
-                       __resources: [
+                       resources: [
                            .copy("foo.txt"),
                            .process("bar.txt"),
                        ]