Include ALL source artifacts in published sources jars

This commit updates the Gradle build to ensure that *all* source
artifacts are including in published 'sources' jars.

To achieve this, we are now assembling 'sources' jars from
'sourceSets.main.allSource' and not explicitly including or excluding
anything by default.

Issue: SPR-12085
(cherry picked from commit 07629a3c7cc9ad677d31d383015410ad3395addc)