config: detect duplicate & internal job names at parse time

Before this PR, config parsing would accept duplicate job names.
`zrepl daemon` would later fail to start with a panic.
But tools like `zrepl configcheck` would pass.

This PR adds a check to ensure job names are unique.

Similarly, internal job names were not being rejected by config
parsing
Move that check to parse-time as well.

internal/config/config_test.go

@@ -45,6 +45,20 @@ func TestSampleConfigsAreParsedWithoutErrors(t *testing.T) {
 
 }
 
+func TestInvalidSampleConfigsFailToParse(t *testing.T) {
+	paths, err := filepath.Glob("./samples/invalid/*/zrepl.yml")
+	require.NoError(t, err, "glob failed")
+	require.NotEmpty(t, paths, "no invalid sample configs found")
+
+	for _, p := range paths {
+		t.Run(p, func(t *testing.T) {
+			_, err := ParseConfig(p)
+			require.Error(t, err, "expected config %s to fail parsing", p)
+			t.Logf("config %s failed as expected: %v", p, err)
+		})
+	}
+}
+
 // template must be a template/text template with a single '{{ . }}' as placeholder for val
 //
 //nolint:deadcode,unused