I need to cross-build a project with both Scala 2.10 and 2.11 (and ultimately 2.12 also). Some packages that were part of 2.10, for example parser-combinators, are packaged independently in 2.11. Thus they are not required to be mentioned in the 2.10 build but are required in 2.11. Furthermore, there may be more than one such package, which are required to use a common version.
I found the documentation re: cross-building on the SBT site to be somewhat lacking in helpfulness here. And, although, there are several StackOverflow Q&As which relate to this subject, I could not find one that addressed this specific point.