Allow registration of exports from an external assembly

[tiktb8]

I'm using a slightly modified version for another project where the DI classes are in a core assembly that my plugin projects reference.

This change allows that by registering the exports from the derived class assembly.

[kip-dk]

Hi jdvachal
I assume you then use ILMerge to get all the assemblies into one single assembly for deployment ?

I prefer avoid the complexity of ILMerge as long as i do not need other libraries.

[tiktb8]

Unfortunately ILMerge is the only solution we've got for dependent assemblies with using dynamics 365 online. The msbuild task for it take some of the headache out but it's not the most glorious solution.