Add installation of libusb-targets.cmake

[Youw]

HIDAPI should be a good example how to do so right.

Related to: #34

In scope of this task:

• add CI/tests for exported targets
• add libusb::include target for convenience
• add libusb::usb-1.0 target for compatibility of package and in-source builds

[Youw]

Maybe need to do #39 first, so CMake package is as much alike to autotools one as possible.