Currently, the Pipeline class offers both const and non-const getSources() methods. We want to minimize the number of non-const calls, and ideally, remove the non-const getSources().

Currently, there are 30 getSources().begin() calls and 13 getSources().rbegin() calls. Replacing some of them with cbegin() and crbegin() will clearly indicate where non-constedness is still intentionally required.

This minor improvement is related to SPM-4106, but it is out of scope of the project.