Add option to emulate virtual environment dependent CodeChecker.

[vodorok]

Only 2/3 of the CodeChecker discovery related functions are tested currently. The custom/user built packages cannot be tested, as those depend on a virutal environment.
This could be emulated as:

• A small modification of the CodeChecker stub, that this venv dependent version only emits the correct responses when there is a certain environment variable set.
• This environment variable could be set in the activate script of the dummy virtual environment.
• The dummy virtual environment should retain the original structure of the genuine.
  This way the virtual environment could be successfully emulated.

[vodorok]

There is no need for virtual environment specification in the plugin, after Standalone packaging got introduced to CodeChecker. #180