fix(static-checks): refactor and improve include order detection

The following changes have been made:

* Adjusted the order in which header files are being located -
  swapped plat_incs and proj_incs, to make sure that the cases
  are prioritized from the most to least specific.
* Removed third_party_incs which had no effect due to a syntax
  error - an if condition checked whether a string is located
  in a list of sets, which always fails. Additionally, includes
  of this type are already being covered by the fallback case.
* Unified include groups into a single ordered dictionary.
* Reworked the logic of the loop that determines the correct
  include group index.
* Removed slashes at the end of some paths.
* Changed the error to a warning and added the word "probably",
  since the detection logic of the script is probabilistic.

Change-Id: I287d9afc3fb753b23e1a213ef0f155d8b0726d46
Signed-off-by: Igor Podgainõi <igor.podgainoi@arm.com>
1 file changed