golangci-lint: enable importas linter to force consistent aliases

This enforces the github.com/containerd/containerd/errdefs package to
be aliased as "cerrdefs". Any other alias (or no alias used) results
in a linting failure:

    integration/container/pause_test.go:9:2: import "github.com/containerd/containerd/errdefs" imported as "c8derrdefs" but must be "cerrdefs" according to config (importas)
        c8derrdefs "github.com/containerd/containerd/errdefs"
        ^

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/validate/golangci-lint.yml

@@ -5,6 +5,7 @@ linters:
     - gosec
     - gosimple
     - govet
+    - importas
     - ineffassign
     - misspell
     - revive
@@ -24,6 +25,16 @@ linters:
       - docs
 
 linters-settings:
+  importas:
+    # Do not allow unaliased imports of aliased packages.
+    no-unaliased: true
+
+    alias:
+      # Enforce alias to prevent it accidentally being used instead of our
+      # own errdefs package (or vice-versa).
+      - pkg: github.com/containerd/containerd/errdefs
+        alias: cerrdefs
+
   govet:
     check-shadowing: false
   depguard: