fix: add @ngtools/webpack to project deps only if

@angular-devkit/build-angular is not a dependency

Revert to adding @ngtools/webpack instead of
@angular-devkit/build-angular because the later is ~150mb bigger and
slows down the {N} cloud builds.
Add @ngtools/webpack to the project dependencies only if
@angular-devkit/build-angular isn't there already. That's because the
@angular-devkit/build-angular already depends on @ngtools/webpack. This will prevent the
plugin from adding multiple instances of @ngtools/webpack to the project
which would cause the build to fail.

related to #571, #569

Author: sis0k0

Diff for: dependencyManager.js

@@ -82,12 +82,29 @@ function addDependency(deps, name, version, force) {
 }
 
 function getRequiredDeps(packageJson) {
-    return isAngular({packageJson}) ?
-        {
-            "@angular-devkit/build-angular": "~0.7.0-rc.0",
-            "@angular/compiler-cli": "~6.1.0-beta.1",
-        } :
-        { };
+    if (!isAngular({packageJson})) {
+        return false;
+    }
+
+    const deps = {
+       "@angular/compiler-cli": "~6.1.0-beta.3",
+    };
+
+    if (!dependsOn(packageJson, "@angular-devkit/build-angular")) {
+        deps["@ngtools/webpack"] = "~6.1.0-rc.0";
+    }
+
+    return deps;
+}
+
+
+function dependsOn(packageJson, package) {
+    if (!packageJson) {
+        return false;
+    }
+
+    return packageJson.dependencies.hasOwnProperty(package) ||
+        packageJson.devDependencies.hasOwnProperty(package);
 }
 
 function showHelperMessages({ newDepsAdded, hasOldDeps }) {