build: Move the Snap recipes

Snapcraft can find its build recipes in 2 places:

* a snap/ directory at the top of the sources;
* a build-aux/snap/ directory, since Snapcraft 3.3;

This commit moves from the former to the latter, so that we can group
all the packaging recipes together.

build-aux/snap/snapcraft.yaml

@@ -60,7 +60,7 @@ apps:
 parts:
   launchers:
     plugin: dump
-    source: snap/local/launchers
+    source: build-aux/snap/local/launchers
     organize:
       '*': bin/