Don't require existence of dependencies not needed for the current --target

[infinity0]

I've been experimenting with converting platform-specific cargo dependencies into platform-specific Debian dependencies. At the moment this is very crude but works in most Real World cases, allowing us to avoid installing winapi etc (and core-foundation, redox-*, fuchsia-*) as build-dependencies when building various crates in Debian. However cargo itself complains about it and fails:

Source: rust-backtrace
Section: rust
Priority: optional
Build-Depends: debhelper (>= 11),
 dh-cargo (>= 8),
 cargo:native <!nocheck>,
 rustc:native <!nocheck>,
 libstd-rust-dev <!nocheck>,
 librust-backtrace-sys-0.1+default-dev (>= 0.1.17~~) <!nocheck>,
 librust-cfg-if-0.1+default-dev <!nocheck>,
 librust-libc-0.2+default-dev <!nocheck>,
 librust-rustc-demangle-0.1+default-dev (>= 0.1.4~~) <!nocheck>,
 librust-winapi-0.3+dbghelp-dev (>= 0.3.3~~) [windows-any] <!nocheck>,
 librust-winapi-0.3+default-dev (>= 0.3.3~~) [windows-any] <!nocheck>,
 librust-winapi-0.3+minwindef-dev (>= 0.3.3~~) [windows-any] <!nocheck>,
 librust-winapi-0.3+processthreadsapi-dev (>= 0.3.3~~) [windows-any] <!nocheck>,
 librust-winapi-0.3+std-dev (>= 0.3.3~~) [windows-any] <!nocheck>,
 librust-winapi-0.3+winnt-dev (>= 0.3.3~~) [windows-any] <!nocheck>
Maintainer: Debian Rust Maintainers <pkg-rust-maintainers@alioth-lists.debian.net>
[..]

The [windows-any] annotation tells most Debian tools that those dependencies can be ignored. When I build it on x86-64 Debian, the Debian build tool does indeed omit installing these build-dependencies. But cargo doesn't like the fact that they're absent:

[..]
   dh_auto_build -O--buildsystem=cargo
   dh_auto_test -O--buildsystem=cargo
error: no matching package named `winapi` found
location searched: registry `https://github.com/rust-lang/crates.io-index`
required by package `backtrace v0.3.9 (file:///<<PKGBUILDDIR>>)`
dh_auto_test: cargo build --verbose --verbose -j4 --target x86_64-unknown-linux-gnu -Zavoid-dev-deps returned exit code 101
make: *** [debian/rules:3: build] Error 101
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2

This seems related to #5655 and #5133, but according to alexcrichton/cargo-vendor#70 seems pretty hard to achieve. We have the necessary infrastructure in place to start taking advantage of it in Debian however, in case this encourages anyone to work on it. Then we won't have to ping everyone to upgrade from winapi 0.2 to 0.3 any more :)

[alexcrichton]

I believe this is the same as #5133 in that the reason Cargo needs these dependencies is to generate a full Cargo.lock. That issue is about producing a minimal Cargo.lock which will release Cargo from needing to do a full graph resolve.