`crate_universe` should render unique files for each dependency

There shouldn't be massive monolithic `BUILD.bazel` files. Each dependency should have it's own build file similar to how `cargo-raze` generates it's output.

Currently from `crate_universe`
```
├── BUILD.bazel
├── WORKSPACE
└── defs.bzl
```

vs something easier to follow

```
├── BUILD.bazel
├── WORKSPACE.bazel
├── defs.bzl
└── crates
├── BUILD.addr2line-0.14.1.bazel
├── BUILD.adler-0.2.3.bazel
...
...
...
├── BUILD.winapi-0.3.9.bazel
├── BUILD.winapi-i686-pc-windows-gnu-0.4.0.bazel
└── BUILD.winapi-x86_64-pc-windows-gnu-0.4.0.bazel
```