Add OS-based dependency specification to Pkg

Here is a "simplest thing that might work" attempt to fix https://github.com/JuliaLang/julia/issues/4323

With this patch, OS-specific dependencies are specified in `requires` as:

```
@windows RPMmd
@linux APT
@osx Homebrew
```

Tested on linux and Windows with this [bogus METADATA](https://github.com/ihnorton/METADATA.jl/commit/af9698be18b5189decdf819ab9fbd1fec30eb1b4). Matched-platform requirement works; unmatched-platform requirement is ignored and doesn't seem to break anything.

cc: @StefanKarpinski