Use imported library, which does not create additional rules, but still
allows attaching arbitraty properties instead of custom targets. This
allows the targets to not appear in the target list of IDEs such as
CLion.
!4452 used setting LINK_LIBRARIES and INTERFACE_LINK_LIBRARIES to link
components built under ESP-IDF build system. However, LINK_LIBRARIES does
not produce behavior same as linking PRIVATE. This MR uses the new
signature for target_link_libraries directly instead. This also moves
setting dependencies during component registration rather than after all
components have been processed.
The consequence is that internally, components have to use the new
signature form as well. This does not affect linking the components to
external targets, such as with idf_as_lib example. This only affects
linking additional libraries to ESP-IDF libraries outside component processing (after
idf_build_process), which is not even possible for CMake<v3.13 as
target_link_libraries is not valid for targets not created in current
directory. See https://cmake.org/cmake/help/v3.13/policy/CMP0079.html#policy:CMP0079
!4452 simplified early expansion by using an early expansion script that
only does one thing: get the public and private requirements for each
component, albeit one by one. This was also dependent on parsing
the command output of the expansion script. This commit makes it so that a list of all
components to be processed to passed to the expansion script, generating a cmake
file that sets each component requirements in one go.
This also makes sure that only components that registered themselves get
included in the final build list.
