Fix pattern match on buildLoaders.

Fixes #1341
Merge pull request #1342 from sbt/wip/fix-build-loaders

Fix pattern match on buildLoaders.
Merge remote-tracking branch 'origin/0.13.5' into wip/merge-0.13.5

Conflicts:
	main/src/main/scala/sbt/Load.scala
Merge pull request #1344 from sbt/wip/merge-0.13.5

Wip/merge 0.13.5