In Mbed OS, each library has an `include/<library>/` subdirectory
containing headers. The recommended way to include a header is
`#include "<library>/<header>.h"` to avoid potential conflicts with
any external modules that have same names of headers.

This is not enforced yet, and both include/ and include/<component>/
are in a library's include paths, to avoid breaking preexisting
Mbed projects that don't follow the recommendation. But code within
Mbed OS should follow it at least.