work around plugin loading multiple times (#805)
work around plugin loading multiple times (#805)

* work around plugin loading multiple times

* resolve comments

superyyrrzz/docfx,LordZoltan/docfx,dotnet/docfx,hellosnow/docfx,hellosnow/docfx,928PJY/docfx,928PJY/docfx,superyyrrzz/docfx,DuncanmaMSFT/docfx,dotnet/docfx,LordZoltan/docfx,superyyrrzz/docfx,LordZoltan/docfx,hellosnow/docfx,pascalberger/docfx,dotnet/docfx,DuncanmaMSFT/docfx,928PJY/docfx,pascalberger/docfx,LordZoltan/docfx,pascalberger/docfx