tpl/tplimpl: Rework template management to get rid of concurrency issues

This more or less completes the simplification of the template handling code in Hugo started in v0.62.

The main motivation was to fix a long lasting issue about a crash in HTML content files  without front matter.

But this commit also comes with a big functional improvement.

As we now have moved the base template evaluation to the build stage we now use the same lookup rules for `baseof` as for `list` etc. type of templates.

This means that in this simple example you can have a `baseof` template for the `blog` section without having to duplicate the others:

```
layouts
├── _default
│   ├── baseof.html
│   ├── list.html
│   └── single.html
└── blog
    └── baseof.html
```

Also, when simplifying code, you often get rid of some double work, as shown in the "site building" benchmarks below.

These benchmarks looks suspiciously good, but I have repeated the below with ca. the same result. Compared to master:

```
name                              old time/op    new time/op    delta
SiteNew/Bundle_with_image-16        13.1ms ± 1%    10.5ms ± 1%  -19.34%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16    13.0ms ± 0%    10.7ms ± 1%  -18.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16      46.4ms ± 2%    43.1ms ± 1%   -7.15%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16            52.2ms ± 2%    47.8ms ± 1%   -8.30%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16        77.9ms ± 1%    70.9ms ± 1%   -9.01%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16      43.0ms ± 0%    37.2ms ± 1%  -13.54%  (p=0.029 n=4+4)
SiteNew/Page_collections-16         58.2ms ± 1%    52.4ms ± 1%   -9.95%  (p=0.029 n=4+4)

name                              old alloc/op   new alloc/op   delta
SiteNew/Bundle_with_image-16        3.81MB ± 0%    2.22MB ± 0%  -41.70%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16    3.60MB ± 0%    2.01MB ± 0%  -44.20%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16      19.3MB ± 1%    14.1MB ± 0%  -26.91%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16            70.7MB ± 0%    69.0MB ± 0%   -2.40%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16        37.1MB ± 0%    31.2MB ± 0%  -15.94%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16      17.6MB ± 0%    10.6MB ± 0%  -39.92%  (p=0.029 n=4+4)
SiteNew/Page_collections-16         25.9MB ± 0%    21.2MB ± 0%  -17.99%  (p=0.029 n=4+4)

name                              old allocs/op  new allocs/op  delta
SiteNew/Bundle_with_image-16         52.3k ± 0%     26.1k ± 0%  -50.18%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16     52.3k ± 0%     26.1k ± 0%  -50.16%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16        336k ± 1%      269k ± 0%  -19.90%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16              422k ± 0%      395k ± 0%   -6.43%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16          401k ± 0%      313k ± 0%  -21.79%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16        247k ± 0%      143k ± 0%  -42.17%  (p=0.029 n=4+4)
SiteNew/Page_collections-16           282k ± 0%      207k ± 0%  -26.55%  (p=0.029 n=4+4)
```

Fixes #6716
Fixes #6760
Fixes #6768
Fixes #6778

common/herrors/errors.go

@@ -15,11 +15,14 @@
 package herrors
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"os"
+	"runtime"
 	"runtime/debug"
+	"strconv"
 
 	_errors "github.com/pkg/errors"
 )
@@ -33,13 +36,13 @@ type stackTracer interface {
 	StackTrace() _errors.StackTrace
 }
 
-// PrintStackTrace prints the error's stack trace to stdoud.
-func PrintStackTrace(err error) {
-	FprintStackTrace(os.Stdout, err)
+// PrintStackTraceFromErr prints the error's stack trace to stdoud.
+func PrintStackTraceFromErr(err error) {
+	FprintStackTraceFromErr(os.Stdout, err)
 }
 
-// FprintStackTrace prints the error's stack trace to w.
-func FprintStackTrace(w io.Writer, err error) {
+// FprintStackTraceFromErr prints the error's stack trace to w.
+func FprintStackTraceFromErr(w io.Writer, err error) {
 	if err, ok := err.(stackTracer); ok {
 		for _, f := range err.StackTrace() {
 			fmt.Fprintf(w, "%+s:%d\n", f, f)
@@ -47,6 +50,13 @@ func FprintStackTrace(w io.Writer, err error) {
 	}
 }
 
+// PrintStackTrace prints the current stacktrace to w.
+func PrintStackTrace(w io.Writer) {
+	buf := make([]byte, 1<<16)
+	runtime.Stack(buf, true)
+	fmt.Fprintf(w, "%s", buf)
+}
+
 // Recover is a helper function that can be used to capture panics.
 // Put this at the top of a method/function that crashes in a template:
 //     defer herrors.Recover()
@@ -56,7 +66,16 @@ func Recover(args ...interface{}) {
 		args = append(args, "stacktrace from panic: \n"+string(debug.Stack()), "\n")
 		fmt.Println(args...)
 	}
+}
 
+// Get the current goroutine id. Used only for debugging.
+func GetGID() uint64 {
+	b := make([]byte, 64)
+	b = b[:runtime.Stack(b, false)]
+	b = bytes.TrimPrefix(b, []byte("goroutine "))
+	b = b[:bytes.IndexByte(b, ' ')]
+	n, _ := strconv.ParseUint(string(b), 10, 64)
+	return n
 }
 
 // ErrFeatureNotAvailable denotes that a feature is unavailable.

common/types/types.go

@@ -21,6 +21,12 @@ import (
 	"github.com/spf13/cast"
 )
 
+// RLocker represents the read locks in sync.RWMutex.
+type RLocker interface {
+	RLock()
+	RUnlock()
+}
+
 // KeyValueStr is a string tuple.
 type KeyValueStr struct {
 	Key   string