BUG: Reset ITK_MODULES_REQUESTED to all modules with multiple calls.

This generates more expected behaviour when

  find_package(ITK COMPONENTS X Y Z)

followed by

  find_package(ITK)

is called.  When no components are specified in subsequent calls, reset
ITK_MODULES_REQUESTED to all modules enabled.

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

Change-Id: I847f1ed2a8912fb5f9b70d1e2ff3a81794350d14
COMP: Add ITK_SOURCE_DIR to doxygen EXAMPLE_PATH.

For warnings like

  <Examples/RegistrationITKv4/ImageRegistration4.py>:3: warning: included file
  Examples/RegistrationITKv4/ImageRegistration4.py is not found. Check your
  EXAMPLE_PATH

Change-Id: If39724b00dd05a65e8326948d3e8a06c817d6fec
Merge topic 'default-modules-requested'

e56cf380 BUG: Reset ITK_MODULES_REQUESTED to all modules with multiple calls.
Merge topic 'doxygen-warnings'

a4f122de COMP: Add ITK_SOURCE_DIR to doxygen EXAMPLE_PATH.