Use default-target to build package

[onur]

This is my solution for default-target. It is using default-target metadata option to build package in first place and placing default build to root of the crate documentation directory.

For example winapi will always be build with x86_64-pc-windows-msvc target first and documentation crated with this target will be available in docs.rs/winapi/$version/winapi/.

[QuietMisdreavus]

This looks good! As you mentioned, by doing the initial build on the default-target, this will ensure that crates that only build on that target (or are practically only available on a certain target even if they build everywhere, like winapi) are checked properly before giving up on building their documentation. I have one comment, but since it matches what the old code does, you can merge this without changing it.

Are you planning on rebuilding crates that already have a default-target set? The reason i went with a redirect for my version was because winapi was already published with that set, so it would work without having to rebuild it.