We want to increase the encapsulation of the Pipeline class and reduce exposure of its inner constructs.

Currently, Pipeline::getSources() has around 450 call sites in master. 114 of them is for pipeline->getSources().size(), and 26 is for pipeline->getSources().empty().

We want to introduce force-inlined Pipeline::size() and Pipeline::empty() to increase code readability, and as a first step to reduce the total number of Pipeline::getSources() calls.

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