Set golangci-lint --exclude-use-default=false

This enables some checks that golangci-lint disables by default, such as
checking for package comments. If it turns out that any of these checks
are too noisy, we can re-enable them. However, right now, it's causing
lint checks to fail when we try to import our code into Google (where
these checks are not disabled).

See https://github.com/golangci/golangci-lint/blob/v1.21.0/README.md#command-line-options

.golangci.yaml

@@ -31,3 +31,7 @@ linters:
     - varcheck
     # TODO(gbelvin): write license linter and commit to upstream.
     # ./scripts/check_license.sh is run by ./scripts/presubmit.sh
+
+issues:
+    # Don't turn off any checks by default. We can do this explicitly if needed.
+    exclude-use-default: false