Prepare for Goldmark

This commmit prepares for the addition of Goldmark as the new Markdown renderer in Hugo.

This introduces a new `markup` package with some common interfaces and each implementation in its own package.

See #5963

markup/internal/external.go

@@ -0,0 +1,52 @@
+package internal
+
+import (
+	"bytes"
+	"os/exec"
+	"strings"
+
+	"github.com/gohugoio/hugo/markup/converter"
+)
+
+func ExternallyRenderContent(
+	cfg converter.ProviderConfig,
+	ctx converter.DocumentContext,
+	content []byte, path string, args []string) []byte {
+
+	logger := cfg.Logger
+	cmd := exec.Command(path, args...)
+	cmd.Stdin = bytes.NewReader(content)
+	var out, cmderr bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &cmderr
+	err := cmd.Run()
+	// Most external helpers exit w/ non-zero exit code only if severe, i.e.
+	// halting errors occurred. -> log stderr output regardless of state of err
+	for _, item := range strings.Split(cmderr.String(), "\n") {
+		item := strings.TrimSpace(item)
+		if item != "" {
+			logger.ERROR.Printf("%s: %s", ctx.DocumentName, item)
+		}
+	}
+	if err != nil {
+		logger.ERROR.Printf("%s rendering %s: %v", path, ctx.DocumentName, err)
+	}
+
+	return normalizeExternalHelperLineFeeds(out.Bytes())
+}
+
+// Strips carriage returns from third-party / external processes (useful for Windows)
+func normalizeExternalHelperLineFeeds(content []byte) []byte {
+	return bytes.Replace(content, []byte("\r"), []byte(""), -1)
+}
+
+func GetPythonExecPath() string {
+	path, err := exec.LookPath("python")
+	if err != nil {
+		path, err = exec.LookPath("python.exe")
+		if err != nil {
+			return ""
+		}
+	}
+	return path
+}