Avoid reading from Viper for path and URL funcs

The gain, given the "real sites benchmark" below, is obvious:

```
benchmark           old ns/op       new ns/op       delta
BenchmarkHugo-4     14497594101     13084156335     -9.75%

benchmark           old allocs     new allocs     delta
BenchmarkHugo-4     57404335       48282002       -15.89%

benchmark           old bytes       new bytes      delta
BenchmarkHugo-4     9933505624      9721984424     -2.13%
```

Fixes #2495

helpers/pathspec_test.go

@@ -0,0 +1,45 @@
+// Copyright 2016-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package helpers
+
+import (
+	"testing"
+
+	"github.com/spf13/viper"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewPathSpecFromConfig(t *testing.T) {
+	viper.Set("disablePathToLower", true)
+	viper.Set("removePathAccents", true)
+	viper.Set("uglyURLs", true)
+	viper.Set("multilingual", true)
+	viper.Set("defaultContentLanguageInSubdir", true)
+	viper.Set("defaultContentLanguage", "no")
+	viper.Set("currentContentLanguage", NewLanguage("no"))
+	viper.Set("canonifyURLs", true)
+	viper.Set("paginatePath", "side")
+
+	pathSpec := NewPathSpecFromConfig(viper.GetViper())
+
+	require.True(t, pathSpec.canonifyURLs)
+	require.True(t, pathSpec.defaultContentLanguageInSubdir)
+	require.True(t, pathSpec.disablePathToLower)
+	require.True(t, pathSpec.multilingual)
+	require.True(t, pathSpec.removePathAccents)
+	require.True(t, pathSpec.uglyURLs)
+	require.Equal(t, "no", pathSpec.defaultContentLanguage)
+	require.Equal(t, "no", pathSpec.currentContentLanguage.Lang)
+	require.Equal(t, "side", pathSpec.paginatePath)
+}