staticcheck (#313)

* CI: use staticcheck for linting

This commit switches the linter for Go code from golint to staticcheck.
Golint has been deprecated since last year and staticcheck is a
recommended replacement.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

* revendor

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

* cmd,pkg: fix lint warnings

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

vendor/honnef.co/go/tools/analysis/facts/directives.go

@@ -0,0 +1,20 @@
+package facts
+
+import (
+	"reflect"
+
+	"golang.org/x/tools/go/analysis"
+	"honnef.co/go/tools/analysis/lint"
+)
+
+func directives(pass *analysis.Pass) (interface{}, error) {
+	return lint.ParseDirectives(pass.Files, pass.Fset), nil
+}
+
+var Directives = &analysis.Analyzer{
+	Name:             "directives",
+	Doc:              "extracts linter directives",
+	Run:              directives,
+	RunDespiteErrors: true,
+	ResultType:       reflect.TypeOf([]lint.Directive{}),
+}