Misc config loading fixes

The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:

```
BenchmarkDefaultConfigProvider/Viper-16         	  252418	      4546 ns/op	    2720 B/op	      30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16        	  450756	      2651 ns/op	    1008 B/op	       6 allocs/op
```

Fixes #8633
Fixes #8618
Fixes #8630
Updates #8591
Closes #6680
Closes #5192

4 files changed, 11 insertions, 23 deletions

diff --git a/helpers/content_test.go b/helpers/content_test.go
index 573c1c90d..515b788f1 100644
--- a/helpers/content_test.go
+++ b/helpers/content_test.go
@@ -19,12 +19,11 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/gohugoio/hugo/config"
 	"github.com/spf13/afero"
 
 	"github.com/gohugoio/hugo/common/loggers"
 
-	"github.com/spf13/viper"
-
 	qt "github.com/frankban/quicktest"
 )
 
@@ -103,7 +102,7 @@ func TestBytesToHTML(t *testing.T) {
 }
 
 func TestNewContentSpec(t *testing.T) {
-	cfg := viper.New()
+	cfg := config.New()
 	c := qt.New(t)
 
 	cfg.Set("summaryLength", 32)
diff --git a/helpers/general_test.go b/helpers/general_test.go
index 518188c17..bfabcbef4 100644
--- a/helpers/general_test.go
+++ b/helpers/general_test.go
@@ -19,7 +19,7 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/spf13/viper"
+	"github.com/gohugoio/hugo/config"
 
 	"github.com/gohugoio/hugo/common/loggers"
 
@@ -29,7 +29,7 @@ import (
 
 func TestResolveMarkup(t *testing.T) {
 	c := qt.New(t)
-	cfg := viper.New()
+	cfg := config.New()
 	spec, err := NewContentSpec(cfg, loggers.NewErrorLogger(), afero.NewMemMapFs())
 	c.Assert(err, qt.IsNil)
 
diff --git a/helpers/path_test.go b/helpers/path_test.go
index e1ac90b2f..c95951832 100644
--- a/helpers/path_test.go
+++ b/helpers/path_test.go
@@ -31,7 +31,6 @@ import (
 
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/spf13/afero"
-	"github.com/spf13/viper"
 )
 
 func TestMakePath(t *testing.T) {
@@ -490,8 +489,6 @@ func TestExists(t *testing.T) {
 }
 
 func TestAbsPathify(t *testing.T) {
-	defer viper.Reset()
-
 	type test struct {
 		inPath, workingDir, expected string
 	}
@@ -511,7 +508,6 @@ func TestAbsPathify(t *testing.T) {
 	}
 
 	for i, d := range data {
-		viper.Reset()
 		// todo see comment in AbsPathify
 		ps := newTestDefaultPathSpec("workingDir", d.workingDir)
 
diff --git a/helpers/testhelpers_test.go b/helpers/testhelpers_test.go
index 4a6ebd0b1..7d63e4d88 100644
--- a/helpers/testhelpers_test.go
+++ b/helpers/testhelpers_test.go
@@ -2,24 +2,24 @@ package helpers
 
 import (
 	"github.com/gohugoio/hugo/common/loggers"
+	"github.com/gohugoio/hugo/config"
 	"github.com/spf13/afero"
-	"github.com/spf13/viper"
 
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/modules"
 )
 
-func newTestPathSpec(fs *hugofs.Fs, v *viper.Viper) *PathSpec {
+func newTestPathSpec(fs *hugofs.Fs, v config.Provider) *PathSpec {
 	l := langs.NewDefaultLanguage(v)
 	ps, _ := NewPathSpec(fs, l, nil)
 	return ps
 }
 
 func newTestDefaultPathSpec(configKeyValues ...interface{}) *PathSpec {
-	v := viper.New()
+	v := config.New()
 	fs := hugofs.NewMem(v)
-	cfg := newTestCfgFor(fs)
+	cfg := newTestCfg()
 
 	for i := 0; i < len(configKeyValues); i += 2 {
 		cfg.Set(configKeyValues[i].(string), configKeyValues[i+1])
@@ -27,15 +27,8 @@ func newTestDefaultPathSpec(configKeyValues ...interface{}) *PathSpec {
 	return newTestPathSpec(fs, cfg)
 }
 
-func newTestCfgFor(fs *hugofs.Fs) *viper.Viper {
-	v := newTestCfg()
-	v.SetFs(fs.Source)
-
-	return v
-}
-
-func newTestCfg() *viper.Viper {
-	v := viper.New()
+func newTestCfg() config.Provider {
+	v := config.New()
 	v.Set("contentDir", "content")
 	v.Set("dataDir", "data")
 	v.Set("i18nDir", "i18n")
@@ -56,7 +49,7 @@ func newTestCfg() *viper.Viper {
 }
 
 func newTestContentSpec() *ContentSpec {
-	v := viper.New()
+	v := config.New()
 	spec, err := NewContentSpec(v, loggers.NewErrorLogger(), afero.NewMemMapFs())
 	if err != nil {
 		panic(err)