all: move standalone libraries to libs/ subdirectory

The root dir of our repository has grown quite a lot the past few months.

I'd like to make it more clear where the bulk of the engine lives (`src/`) and
also make it more clear which Mach libraries are consumable as standalone projects.

As for the name of this directory, `libs` was my first choice but there's a bit of
a convention of that being external libraries in Zig projects _today_, while these
are libraries maintained as part of Mach in this repository - not external ones.

We will name this directory `libs`, and if we have a need for external libraries
we will use `external` or `deps` for that directory name. I considered other names
such as `components`, `systems`, `modules` (which are bad as they overlap with
major ECS / engine concepts), and it seems likely the official Zig package manager
will break the convention of using a `libs` dir anyway.

Performed via:

```sh
mkdir libs/
git mv freetype libs/
git mv basisu libs/
git mv gamemode libs/
git mv glfw libs/
git mv gpu libs/
git mv gpu-dawn libs/
git mv sysaudio libs/
git mv sysjs libs/
git mv ecs libs/
```

git-subtree-dir: glfw
git-subtree-mainline: 0d5b853443
git-subtree-split: 572d1144f11b353abdb64fff828b25a4f0fbb7ca

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

git mv ecs libs/

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

.gitmodules

@@ -1,10 +1,10 @@
 [submodule "gpu-dawn/libs/dawn"]
-	path = gpu-dawn/libs/dawn
+	path = libs/gpu-dawn/libs/dawn
 	url = https://github.com/hexops/dawn.git
 	shallow = true
 	branch = "generated-2022-08-06"
 [submodule "gpu-dawn/libs/DirectXShaderCompiler"]
-	path = gpu-dawn/libs/DirectXShaderCompiler
+	path = libs/gpu-dawn/libs/DirectXShaderCompiler
 	url = https://github.com/hexops/DirectXShaderCompiler
 	shallow = true
 	branch = "mach"
@@ -17,19 +17,19 @@
 	url = https://github.com/slimsag/zigimg
 	branch = zig-master
 [submodule "freetype/upstream"]
-	path = freetype/upstream
+	path = libs/freetype/upstream
 	url = https://github.com/hexops/freetype
 [submodule "tools/libs/apple_pie"]
 	path = tools/libs/apple_pie
 	url = https://github.com/Luukdegram/apple_pie
 [submodule "glfw/upstream"]
-	path = glfw/upstream
+	path = libs/glfw/upstream
 	url = https://github.com/hexops/glfw
 [submodule "sysaudio/upstream"]
-	path = sysaudio/upstream
+	path = libs/sysaudio/upstream
 	url = https://github.com/hexops/soundio
 [submodule "basisu/upstream"]
-	path = basisu/upstream
+	path = libs/basisu/upstream
 	url = https://github.com/hexops/basisu
 [submodule "examples/image-blur/assets"]
 	path = examples/image-blur/assets