migrate CI to GitHub Actions

use codecov.io to report on code coverage.
fix a few issues raised by golangci-lint

tmpl_test.go

@@ -40,7 +40,7 @@ func TestFetchTemplate(t *testing.T) {
 	}{
 		// custom template files
 		{
-			"non-existant template file",
+			"non-existent template file",
 			"",
 			"/does/not/exist",
 			spdxOff,
@@ -62,7 +62,7 @@ func TestFetchTemplate(t *testing.T) {
 			"",
 			spdxOff,
 			"",
-			errors.New(`unknown license: "unknown". Include the '-s' flag to request SPDX style headers using this license.`),
+			errors.New(`unknown license: "unknown". Include the '-s' flag to request SPDX style headers using this license`),
 		},
 
 		// pre-defined license templates, no SPDX