Add /config dir support

This commit adds support for a configuration directory (default `config`). The different pieces in this puzzle are:

* A new `--environment` (or `-e`) flag. This can also be set with the `HUGO_ENVIRONMENT` OS environment variable. The value for `environment` defaults to `production` when running `hugo` and `development` when running `hugo server`. You can set it to any value you want (e.g. `hugo server -e "Sensible Environment"`), but as it is used to load configuration from the file system, the letter case may be important. You can get this value in your templates with `{{ hugo.Environment }}`.
* A new `--configDir` flag (defaults to `config` below your project). This can also be set with `HUGO_CONFIGDIR` OS environment variable.

If the `configDir` exists, the configuration files will be read and merged on top of each other from left to right; the right-most value will win on duplicates.

Given the example tree below:

If `environment` is `production`, the left-most `config.toml` would be the one directly below the project (this can now be omitted if you want), and then `_default/config.toml` and finally `production/config.toml`. And since these will be merged, you can just provide the environment specific configuration setting in you production config, e.g. `enableGitInfo = true`. The order within the directories will be lexical (`config.toml` and then `params.toml`).

```bash
config
├── _default
│   ├── config.toml
│   ├── languages.toml
│   ├── menus
│   │   ├── menus.en.toml
│   │   └── menus.zh.toml
│   └── params.toml
├── development
│   └── params.toml
└── production
    ├── config.toml
    └── params.toml
```

Some configuration maps support the language code in the filename (e.g. `menus.en.toml`): `menus` (`menu` also works) and `params`.

Also note that the only folders with "a meaning" in the above listing is the top level directories below `config`. The `menus` sub folder is just added for better organization.

We use `TOML` in the example above, but Hugo also supports `JSON` and `YAML` as configuration formats. These can be mixed.

Fixes #5422

1 files changed, 6 insertions, 4 deletions

diff --git a/hugolib/hugo_sites.go b/hugolib/hugo_sites.go
index 0bb3b4362..5e75135c0 100644
--- a/hugolib/hugo_sites.go
+++ b/hugolib/hugo_sites.go
@@ -21,6 +21,8 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/gohugoio/hugo/config"
+
 	"github.com/gohugoio/hugo/publisher"
 
 	"github.com/gohugoio/hugo/common/herrors"
@@ -361,14 +363,14 @@ func (h *HugoSites) resetLogs() {
 	}
 }
 
-func (h *HugoSites) createSitesFromConfig() error {
+func (h *HugoSites) createSitesFromConfig(cfg config.Provider) error {
 	oldLangs, _ := h.Cfg.Get("languagesSorted").(langs.Languages)
 
 	if err := loadLanguageSettings(h.Cfg, oldLangs); err != nil {
 		return err
 	}
 
-	depsCfg := deps.DepsCfg{Fs: h.Fs, Cfg: h.Cfg}
+	depsCfg := deps.DepsCfg{Fs: h.Fs, Cfg: cfg}
 
 	sites, err := createSitesFromConfig(depsCfg)
 
@@ -412,9 +414,9 @@ func (h *HugoSites) toSiteInfos() []*SiteInfo {
 type BuildCfg struct {
 	// Reset site state before build. Use to force full rebuilds.
 	ResetState bool
-	// Re-creates the sites from configuration before a build.
+	// If set, we re-create the sites from the given configuration before a build.
 	// This is needed if new languages are added.
-	CreateSitesFromConfig bool
+	NewConfig config.Provider
 	// Skip rendering. Useful for testing.
 	SkipRender bool
 	// Use this to indicate what changed (for rebuilds).