Individual libraries' `target_h` stub headers have now all been moved
from `mbed-headers-base` to `mbed-headers-<library>`.

Note: Even though headers previously in `target_h` are technically
stubs/fakes too, they are used by not only unit tests but also regular
libraries when compiled for unit tests, because no target-specific HAL
implementation exists in this case. In order for regular library
sources to pick up `target_h` headers, those headers must

* have the same names as regular headers
* appear first in include paths

This is why those headers are part of `mbed-headers-<library>` and not
`mbed-stubs-<library>`. Before this refactoring, `mbed-headers-base`
was the first in unit tests' include paths.

Previously events fakes as part of UNITTESTS/fakes, this PR moves
events fakes to events double directory to make events stubs
to be self-contained.

Previously the events stub library depended on `mbed-headers`, which
is a collection of all available headers in mbed-os. To make it easier
to separate the library, only depend on the headers we're using.

Move events stubs headers to the mbed-os/events/tests/UNITTESTS/double
directory and update double CMake to include stub headers

Move the header-only mbed-headers-events library the unit test stubs
depend on into the events component directory. This makes the events
stubs more self-contained and improves the composition of the library.

Move the events stub into the events component directory.
so we can avoid duplicating the mbed-os source tree in a central
UNITTESTS folder.