Configure directory structure of $EXT_BUILD_DEPS

When I pass a few other Bazel dependencies into `cmake_external(.. deps = [...])`, all of the headers seem to be copied into $`EXT_BUILD_DEPS/include` and libraries to `$EXT_BUILD_DEPS/lib`. However, the directory structure is destroyed. If the dependency creates `external/org_dependency/include/core/platform/utils/foo.h`, then this is just symlinked as `$EXT_BUILD_DEPS/include/foo.h`. This creates a problem because my source code that I'm trying to compile wants to `#include "core/platform/utils/foo.h"`. And I really can't use $EXT_BUILD_DEPS at all because another dependency decided to create their own time.h which is creating all sorts of linker errors, as you could imagine.

Wondering if there is a way to preserve dependency structure.

Right now my workaround, which isn't quite working, is to use flags like:

```
"CMAKE_CXX_FLAGS": "-I $EXT_BUILD_ROOT/bazel-out/k8-opt/bin/external/org_tensorflow/ \
-I $EXT_BUILD_ROOT/external/org_tensorflow/ \
```

Start by locating the implementation of cmake_external(.. deps = [...]) and how it assembles EXT_BUILD_DEPS/include and EXT_BUILD_DEPS/lib. Reproduce the nested-header case with core/platform/utils/foo.h and verify how the current workaround flags resolve dependencies. Done means dependency directory structure is preserved without collisions such as time.h.

Written by the indexing model from the issue text.