text: Title: Allow resource files and libraries to be imported privately in other resource files
  Body: It could be beneficial if users could import resource files and libraries in a "private" way. Currently, resource files are imported in a sort of recursive way where you have access to all keywords that resource file imports as well, as described in the user guide [here](http://robotframework.org/robotframework/latest/RobotFrameworkUserGuide.html#resource-files). This can be unhelpful when you are abstracting a library or resource such that other contributors should not make direct calls to the underlying libraries.
  
  This was especially important to me on a recent project where I was refactoring a test suite to be able choose whether to run with Browser library or SeleniumLibrary to compare their pros and cons. You could pick which browser control to use but writing a test that made direct keyword calls to the opposite library would cause that control to crash out. Using an abstracted resource file allows you to pick which library to use on the fly, however with recursive importation, a contributor could still mistakenly call a non-abstracted keyword from one or the other library and cause the opposing library crash the test when they reach the unexpected keyword.
  
  In more biased terms, private importation of libraries and resources feels like a much more familiar mode for project structure for me given that is how python seems to handle resource/library importation.