bug#25281: rust importer duplicates input

[David Craven]

Looking at the Cargo.toml file we see this:

[target."cfg(unix)".dependencies]
libc = "0.2.14"
# Compat with older Cargo versions temporarily
[target.x86_64-unknown-linux-gnu.dependencies]
libc = "0.2.14"
[target.i686-unknown-linux-gnu.dependencies]
libc = "0.2.14"
[target.x86_64-apple-darwin.dependencies]
libc = "0.2.14"
[target.i686-apple-darwin.dependencies]
libc = "0.2.14"

I started reimplementing cargos dependency resolution algorithm, but
think now that it's better to use a Cargo.lock file when one is
provided.

When there isn't a cargo lock file it's a library in the sense that it
is a collection of source code that can be reused in a cargo project
that has a Cargo.lock file.

I don't think that this is a issue that needs fixing.