new-build had assumed that each component could only directly depend on one
version of each package.  However, a component can depend on two different
versions of a package by listing it as a build-depends and build-tool-depends
dependency.  The incorrect assumption caused new-build to mix up the two
instances of the dependency when converting the solver's package-based install
plan (SolverInstallPlan) to the component-based install plan
(ElaboratedInstallPlan).  This commit fixes the bug by keeping build-depends and
build-tool-depends dependencies separate between reading them from the
SolverPackage and passing them to
D.Backpack.ConfiguredComponent.toConfiguredComponent.

This commit changes the signature of
D.Backpack.ConfiguredComponent.toConfiguredComponent to allow distinguishing
between the two types of dependencies.