Fixes#4293
Ref #4231, #4065
This fixes the regression on sbt 1.2.0 that displays a lot of warnings about configurations.
The warning was added in #4231 in an attempt to fix#4065. This actually highlights somewhat loose usage of configurations among the builds in the wild, and the limitation on the current slash syntax implementation.
I think we can remove this warning for now, and try to fix#4065 by making slash syntax more robust. In particular, we need to memorize the mapping between the configuration name and Scala identifier across the entire build, and use that in the shell.