Just to add a little bit more around Hans-Jörg’s answer.
Items
.items files are flat. There is no need to import a .items files into another one because anything defined in one .items file can only be defined once anyway. So what you are really after here is subfolders which, as mentioned, it was determined would not be supported by the developers.
Rules
The Rules DSL does not support imports of other Rules files. We can’t create Rules DSL libraries (which is what you are really asking about). However, Groovy, JavaScript, and Jython do not have this limitation and all of these languages are (or will) be supported in both OH 2.5 (through installation of the NGRE) and OH 3 (through installation of language addons, JavaScript is supported by default). These other languages support both importing libraries and structuring your code into subfolders.
And, with Rules DSL, there are other ways to avoid needing to have duplicated code. See Design Pattern: DRY, How Not to Repeat Yourself in Rules DSL.