Use strstr instead of strpos in ClassLoader (4% perf improvement)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [
| License       | MIT
| Doc PR        | 

Using ClassLoader in Drupal 8, using strstr instead of strpos nets 4% perf improvement.
XHPROF diff https://dl.dropboxusercontent.com/u/10201421/diff.html