only load mapper compiler pass if the orm resp phpcr-odm is available

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #139
| License       | MIT
| Doc PR        | -

This is an alternative approach to #139 after i fixed the corebundle compiler pass.

The tests only cover phpcr-odm for now. I played with composer.json to have all orm cases in the tests and see it still works (symfony and doctrine-bundle with compiler pass, old symfony, old symfony and corebundle for the forward compatibility compiler pass)