Evaluate <depend> conditions in CMake variables

[matthew-reynolds]

Adds support for the condition attribute for <*depend> tags in package.xml format 3 (REP014)

#1038 added support for the condition attribute to the <build_type> tag, but it was never added to the other <*depend> tags.

Without this change:

In Noetic (py3), package.xml containing

  <exec_depend condition="$ROS_PYTHON_VERSION == 2">python-whatever</exec_depend>
  <exec_depend condition="$ROS_PYTHON_VERSION == 3">python3-whatever</exec_depend>

yields CMake variable: ${PROJECT_NAME}_EXEC_DEPENDS = "python-whatever;python3-whatever"

After this change:

The same package.xml yields CMake variable: ${PROJECT_NAME}_EXEC_DEPENDS = "python3-whatever"

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If approved, I would also like this to be considered for backport to kinetic & melodic.