feature flag overwrites using crates.io do not work, local overwrites with patch do

With the juice project there is an issue regarding the diamond structured dependency tree.

juice needs coaster-nn and coaster-blas, where coaster-blas and coaster-nn each require coaster for the generic API.
All features flags (in this case, backends which is one of native, cuda, opencl) are passed through from coaster-nn coaster-blas and coaster up to juice. The default set is to incude all 3 features native,cuda,opencl (for all 4 libraries).

If someone now wants to create a binary just linking juice itself with non-default features, the linker still pulls in ALL of the linker flags, -lOpenCL -lcuda -lopenblas.

This only fails if all dependencies are pulled in from crates.io, as soon as local overwrites via [replace] or [patch] are used, the linker flags are as expected - no linkage of -lOpenCL or -lcuda is attempted.

See the below thread for more details:

https://github.com/spearow/juice/issues/38#issuecomment-417949764

Sidenote: I am well aware that in a perfect world coaster would not need to know anything about the backends, but we are not quite there yet :)

Reproduce the diamond dependency case described for juice, coaster-nn, coaster-blas, and coaster using crates.io dependencies, comparing default and selected backend features. Compare linker flags with crates.io resolution versus local [replace] or [patch] overrides; done means non-default builds no longer attempt to link OpenCL, CUDA, or OpenBLAS.