Improvements to recently introduced sourceinfo stuff#500

Merged

jhump merged 3 commits intomasterfrom jh/always-include-source-code-info-if-available

Feb 17, 2022

+45-34

Commits on Feb 17, 2022

desc.LoadFileDescriptor always includes source code info if it's available
jhump
committed
update tests in the face of desc.Load* functions including source code info
jhump
committed
remove errant 'descriptorpb' package alias; add experimental note
jhump
committed