enforce the use of goimports (#248)

* enforce the use of goimports

Instead of enforcing `gofmt`, enforce `goimports`, which also asserts
a separate section for non-builtin packages.

* run `goimports` everywhere

* exclude generated .pb.go files

.github/workflows/gofmt.yml

@@ -23,10 +23,22 @@ jobs:
     - name: Check out code into the Go module directory
       uses: actions/checkout@v1
 
+    - uses: actions/cache@v1
+      with:
+        path: ~/go/pkg/mod
+        key: ${{ runner.os }}-gofmt-${{ hashFiles('**/go.sum') }}
+        restore-keys: |
+          ${{ runner.os }}-gofmt-
+
+    - name: Install goimports
+      run: |
+        go get golang.org/x/tools/cmd/goimports
+        go build golang.org/x/tools/cmd/goimports
+
     - name: gofmt
       run: |
-        if [ "$(find . -iname '*.go' | xargs gofmt -l)" ]
+        if [ "$(find . -iname '*.go' | grep -v '\.pb\.go$' | xargs ./goimports -l)" ]
         then
-          find . -iname '*.go' | xargs gofmt -d
+          find . -iname '*.go' | grep -v '\.pb\.go$' | xargs ./goimports -d
           exit 1
         fi