Make Zend\Loader soft dependency

[Xerkus]

This PR adds new use_zend_loader module manager option that controls if Zend\Loader should be used.
Option is enabled by default. Disabling it removes dependency on zendframework/zend-loader.
Setting it to false will disable both ModuleAutoloader and AutoloaderProvider feature.

Autoloading for vendor modules is already managed by Composer, for most of framework users at least, and application specific modules can be switched as easily.
In fact, letting all the modules be managed by composer provides small performance boost and makes setting up tests easier since application no longer needs to be bootstrapped.

no docs yet

Note: originally i wanted to check if Zend\Loader\ModuleAutoloader present but figured that will lead to too much frustration.

[danizord]

👍

[nuxwin]

We need this enhancement. Right now we must install the zend-loader composer package, else, we get an exception... We use the composer loader for all our classes so here, the zend-loader is not needed.

By the way, either the autoloader provider feature should be disabled by default, either you should explicitely require the zend-loader module in the zend-modulemanager composer.json.

[Xerkus]

@nuxwin I would like that too but it can't be done in 2.x unfortunately

[Xerkus]

@weierophinney i am a bit confused. This PR was intended for 2.x, since develop is 3.x now should i reopen it against master?

[Maks3w]

With the current branch model new features must be target develop branch, no matter what version is under development.

[weierophinney]

Thanks, @Xerkus. I rebased against current develop, fixed up some issues, and am targeting it for the 2.8.0 release.