Move //third_party/glfw to //flutter/third_party/glfw (#777)

As part of eliminating the Flutter buildroot (#67373), we are moving all
third-party dependencies from //third_party to //flutter/third_party.

This will be followed by an engine PR that updates the deps pull GLFW to
//flutter/third_party/glfw and update all targets to point to the new
location.

Once all third-party dependencies have been migrated, tooling and config
will be moved and the buildroot will be eliminated altogether.

Issue: flutter/flutter#136284

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there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
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build/secondary/third_party/glfw/BUILD.gn → build/secondary/flutter/third_party/glfw/BUILD.gn

@@ -4,7 +4,7 @@
 
 import("glfw_args.gni")
 
-_checkout_dir = "//third_party/glfw"
+_checkout_dir = "//flutter/third_party/glfw"
 
 config("relative_glfw_headers") {
   include_dirs = [

build/secondary/third_party/imgui/BUILD.gn

@@ -36,7 +36,7 @@ config("imgui_headers") {
 source_set("imgui_glfw") {
   public_deps = [
     ":imgui",
-    "//third_party/glfw",
+    "//flutter/third_party/glfw",
   ]
 
   public_configs = [ ":imgui_headers" ]