internal/cmd: repurpose cue-ast-print into subcommands

I want to add more helpful internal AST debugging tools for my own use,
such as one to join all the CUE files in an instance into a single file
to speed up my reduction of CUE bugs.

Rather than adding more binaries to ./internal/cmd, rename cue-ast-print
to cue-ast-tool and give it subcommands such as "print".

While here, rewrite "print" to take the same sort of inputs as
cmd/cue via cue/load, for the sake of consistency and flexibility.
Printing a single CUE file or stdin still works, with the only exception
that stdin now works via a "-" argument, like in cmd/cue.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I254eacc7ecbc80a5b363a2c064d82ae2a0dd4c5a
Dispatch-Trailer: {"type":"trybot","CL":1200355,"patchset":1,"ref":"refs/changes/55/1200355/1","targetBranch":"master"}

internal/cmd/cue-ast-tool/main.go

@@ -0,0 +1,75 @@
+// Copyright 2023 The CUE Authors
+//
+// 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.
+
+// cue-ast-print parses a CUE file and prints its syntax tree, for example:
+//
+//	cue-ast-print file.cue
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"cuelang.org/go/cue/load"
+	"cuelang.org/go/internal/astinternal"
+)
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprint(flag.CommandLine.Output(), `
+usage of cue-ast-tool:
+
+  cue-ast-tool print [flags] [inputs]
+
+    Write multi-line Go-like representations of CUE syntax trees.
+
+      -omitempty    omit empty and invalid values
+
+See 'cue help inputs' as well.
+`[1:])
+	}
+
+	if len(os.Args) < 2 {
+		flag.Usage()
+		os.Exit(1)
+	}
+	name, args := os.Args[1], os.Args[2:]
+	switch name {
+	case "print":
+		var cfg astinternal.DebugConfig
+		flag.BoolVar(&cfg.OmitEmpty, "omitempty", false, "")
+		// Note that DebugConfig also has a Filter func, but that doesn't lend itself well
+		// to a command line flag. Perhaps we could provide some commonly used filters,
+		// such as "positions only" or "skip positions".
+		flag.CommandLine.Parse(args)
+
+		// TODO: should we produce output in txtar form for the sake of
+		// more clearly separating the AST for each file?
+		// [ast.File.Filename] already has the full filename,
+		// but as one of the first fields it's not a great separator.
+		insts := load.Instances(flag.Args(), &load.Config{})
+		for _, inst := range insts {
+			if err := inst.Err; err != nil {
+				log.Fatal(errors.Details(err, nil))
+			}
+			for _, file := range inst.Files {
+				out := astinternal.AppendDebug(nil, file, cfg)
+				os.Stdout.Write(out)
+			}
+		}
+	default:
+		flag.Usage()
+	}
+}