Only invoke a given cached partial once

Note that this is backed by a LRU cache (which we soon shall see more usage of), so if you're a heavy user of cached partials it may be evicted and
refreshed if needed. But in most cases every partial is only invoked once.

This commit also adds a timeout (the global `timeout` config option) to make infinite recursion in partials
easier to reason about.

```
name              old time/op    new time/op    delta
IncludeCached-10    8.92ms ± 0%    8.48ms ± 1%   -4.87%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    6.65MB ± 0%    5.17MB ± 0%  -22.32%  (p=0.002 n=6+6)

name              old allocs/op  new allocs/op  delta
IncludeCached-10      117k ± 0%       71k ± 0%  -39.44%  (p=0.002 n=6+6)
```

Closes #4086
Updates #9588

deps/deps.go

@@ -11,6 +11,7 @@ import (
 	"github.com/gohugoio/hugo/cache/filecache"
 	"github.com/gohugoio/hugo/common/hexec"
 	"github.com/gohugoio/hugo/common/loggers"
+	"github.com/gohugoio/hugo/common/types"
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/config/security"
 	"github.com/gohugoio/hugo/helpers"
@@ -298,11 +299,14 @@ func New(cfg DepsCfg) (*Deps, error) {
 
 	sp := source.NewSourceSpec(ps, nil, fs.Source)
 
-	timeoutms := cfg.Language.GetInt("timeout")
-	if timeoutms <= 0 {
-		timeoutms = 3000
+	timeout := 30 * time.Second
+	if cfg.Cfg.IsSet("timeout") {
+		v := cfg.Cfg.Get("timeout")
+		d, err := types.ToDurationE(v)
+		if err == nil {
+			timeout = d
+		}
 	}
-
 	ignoreErrors := cast.ToStringSlice(cfg.Cfg.Get("ignoreErrors"))
 	ignorableLogger := loggers.NewIgnorableLogger(logger, ignoreErrors...)
 
@@ -329,7 +333,7 @@ func New(cfg DepsCfg) (*Deps, error) {
 		BuildClosers:            &Closers{},
 		BuildState:              buildState,
 		Running:                 cfg.Running,
-		Timeout:                 time.Duration(timeoutms) * time.Millisecond,
+		Timeout:                 timeout,
 		globalErrHandler:        errorHandler,
 	}