Use strstr instead of strpos
merged branch larowlan/master (PR #8081)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #8081).

Discussion
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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

Commits
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25d7b90 [ClassLoader] Use strstr instead of strpos