fix(@angular-devkit/build-angular): normalize long-form asset option output to relative path

The output option for the long-form of the assets build option should be a relative path based
from the output path of the application. However, a rooted path was also considered relative
to the output path. To avoid two different ways of representing the path throughout the build
system. The output path is now normalized to a relative path at the beginning of the build
process.

(cherry picked from commit 4d8d489)

Author: clydin

packages/angular_devkit/build_angular/src/utils/normalize-asset-patterns.ts

@@ -67,6 +67,8 @@ export function normalizeAssetPatterns(
       const output = path.relative(resolvedSourceRoot, path.resolve(workspaceRoot, input));
 
       assetPattern = { glob, input, output };
+    } else {
+      assetPattern.output = path.join('.', assetPattern.output);
     }
 
     if (assetPattern.output.startsWith('..')) {