[Fix] contrib.glfw3 overrides built-in GLFW (in cache)

[ypujante]

As I was working on another port, something that should not be working was working... so I investigated and found an issue. The contrib port ended up overriding the built-in GLFW in the cache.

After this fix it doesn't happen anymore:

# ls -la cache/sysroot/include/GLFW
-rw-r--r--@ 1 ypujante  wheel  215860 Mar 21 12:54 glfw3.h
# ls -la cache/sysroot/include/contrib.glfw3/GLFW
-rw-r--r--@ 1 ypujante  wheel    6219 Mar 21 12:54 emscripten_glfw3.h
-rw-r--r--@ 1 ypujante  wheel  241826 Mar 21 12:54 glfw3.h

As you can see after the fix the 2 directories contain different versions of glfw3.h (which is expected since as you know I upgraded the contrib port to GLFW 3.4).

When you run the emcc command, you can see that the search path is set properly:

/usr/local/emsdk/emscripten/main/emcc --verbose --use-port=contrib.glfw3 --shell-file=shell.html main.cpp -o build/index.html
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/local/emsdk/emscripten/main/cache/sysroot/include/contrib.glfw3
 /usr/local/emsdk/emscripten/main/cache/sysroot/include/fakesdl
 /usr/local/emsdk/emscripten/main/cache/sysroot/include/compat
 /usr/local/emsdk/emscripten/main/cache/sysroot/include/c++/v1
 /usr/local/emsdk/upstream/lib/clang/19/include
 /usr/local/emsdk/emscripten/main/cache/sysroot/include

So when a source code includes

#include <GLFW/glfw3.h>

then it will pick up the right one under cache/sysroot/include/contrib.glfw3 (and not the one under cache/sysroot/include which is last in the list)

This fix requires a minor change in ports.install_headers to allow for nested folders (which BTW, I also need for another project I am working on)