Import GTK in preparation for rules_python

We currently import GTK libraries as part of the "python_gtk" repo.
That pulls in GTK, but also Python and a bunch of pip packages.

This patch makes it so that GTK is pulled in via a dedicated repo.
That allows us to have dedicated repos for pip packages via
rules_python.

Future patches will make use of the new GTK import.

I had to modify `download_packages.py` in order to ensure that `libc6`
really got excluded from the final tarball. On James' personal
machines, he observed stack smashing crashes when `libc` is in the
tarball. The `force_includes` usage pulled in `libc6` even though we
don't want it to.

Signed-off-by: Philipp Schrader <philipp.schrader@gmail.com>
Change-Id: Ia3ecf51e4bd4c7aa0e72646e4c2af738f6086c01
diff --git a/debian/gtk_runtime.BUILD b/debian/gtk_runtime.BUILD
new file mode 100644
index 0000000..e529782
--- /dev/null
+++ b/debian/gtk_runtime.BUILD
@@ -0,0 +1,12 @@
+filegroup(
+    name = "gtk_runtime",
+    srcs = glob([
+        "etc/**",
+        "lib/x86_64-linux-gnu/**/*.so*",
+        "usr/lib/**/*.so*",
+        "usr/lib/x86_64-linux-gnu/**/*",
+        "usr/share/font*/**",
+        "usr/share/gir-1.0/**",
+    ]),
+    visibility = ["//visibility:public"],
+)