Separate lint levels for multiple versions when the crate is dev- or build-dep'd vs. normal deps.

[nlewycky]

Is your feature request related to a problem? Please describe.
We minimize multiple dependencies in our codebase as part of minimizing the size of our final binary. It'd be great if we could use cargo deny for this, but presently it issues errors on different versions of cfg-if crate which is only a dev-dependency.

Describe the solution you'd like
Separate lint levels for multiple-versions that are going to be included in the build vs. multiple versions that are dev/build only.

The fact it's a dev-dependency are already included in the printout of the dependency structure when there is a conflict.

Describe alternatives you've considered
Presently the workaround is to use skip to skip the whole crate as part of duplicate detection. This is OK to get started but not great long term.

[teor2345]

I just posted bug #469, where this issue seems to get much worse in cargo deny 0.13.0.