feat(third_party): separate nixpkgs whitelist to allow more owners

The exposed package list has to be changed/amended quite frequently,
every time somebody wants to use a package not yet in that list and
thus has to whitelist it here.

This effectively requires a superowner review every single time, which
is an unreasonable blocker for many CLs.

I thus propose moving the list into a separate file (I called it
`nixpkgs-whitelist.nix` which is more descriptive than `exposed.nix`
and letting anybody add themselves to the OWNERS on that file.

Change-Id: Ied8bac066e4b9a91ddd642db805fe33dc37872c9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2323
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>

third_party/OWNERS

@@ -0,0 +1,9 @@
+inherited: true
+
+matchers:
+  # The nixpkgs whitelist needs to be changed every time we reference
+  # a new attribute, so every commiter should be able to change it,
+  # otherwise we create undue blockers on superowners.
+  - exact: nixpkgs-exposed.nix
+    owners:
+      - Profpatsch