This is a much simpler solution for solving hexops/mach#584
1. We continue using submodules everywhere (at least in the Mach codebase.)
2. `dawn` and `DirectXShaderCompiler` (the only two unwiedly submodules that are not needed by default since we use binary builds) are kicked out of the tree.
3. If you specify `-Ddawn-from-source=true`, `zig build` handles cloning those dependencies for you (using `git clone`, not as submodules.)
Signed-off-by: Stephen Gutekanst <stephen@hexops.com>
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