Defer resolving configurations until inserted into dict

[stotko]

The full build and stub generation paths are resolved when initializing a Config object. However, this is too early to draw any associations to the intended module name used for that extension. This restricts picking a build directory name based on only the project name and other auxiliary information. However, the module name would provide a better and more intuitive means in this regard, but could not be used so far. Defer resolving the configuration until it is inserted into the module dictionary. This not only enables leveraging the module name, but would also allow for further sanity checks, e.g. when overriding an existing configuration.