CONTRIB: move some dev-specific tools to dev/

The following directories were moved from contrib/ to dev/ to make their
use case a bit clearer. In short, only developers are expected to ever
go there. The makefile was updated to build and clean from these ones.

base64/  flags/  hpack/  plug_qdisc/  poll/  tcploop/  trace/

.github/workflows/contrib.yml

@@ -13,12 +13,12 @@ jobs:
     - name: Compile contrib/halog/halog
       run: |
         make contrib/halog/halog
-    - name: Compile contrib/debug/flags
+    - name: Compile dev/flags/flags
       run: |
-        make contrib/debug/flags
-    - name: Compile contrib/poll/poll
+        make dev/flags/flags
+    - name: Compile dev/poll/poll
       run: |
-        make contrib/poll/poll
-    - name: Compile contrib/hpack
+        make dev/poll/poll
+    - name: Compile dev/hpack
       run: |
-        make -C contrib/hpack
+        make -C dev/hpack