Re-enable basic object management for 3rd party packages

[bwaidelich]

Description

Flow used to be able to proxy classes of 3rd party packages (it used to be the default until we introduced the includeClasses Setting). Apparently this is not used a lot, but it can be quite nice in order to use DI with other packages for example.
Since #1154 this is no longer possible since the PackageManager doesn't know about 3rd party packages.

Steps to Reproduce

Use the includeClasses setting to enable object management for a (non-neos) package. Try to wire some of the packages via Objects.yaml

Expected behavior

Wiring and implementation should be configurable via Objects.yaml

Actual behavior

The 3rd party classes are not proxied, custom object management is ignored.

Affected Versions

Flow:

dev-master (will be in the next release unless we change it)

[bwaidelich]

I suggest to adjust CompileTimeObjectManager::registerClassFiles() to load the explicitly included classes. I'm not sure how much work that is, but @kitsunet wants to look into it.
Otherwise we should at least fix the documentation