Overwrite module defined patterns on theme layer

[steffenr]

We want to build "kind-of" base components/ ui_patterns in a module. Those patterns can be used "as is" or can be overridden on theme-layer. Best example for such a usecase would be a slider - default would be a slick slider. The slider is fine for most cases, but in some special cases, we want to use a custom library for the slider in our theme. Instead of using a hook alter function or creating a new pattern, the theme-dev would just duplicate the pattern in the theme and change the libraries definition.

[ademarco]

@steffenr could you please rebase your PR on latest 8.x-1.x? We should also add tests for this.