feat(internal/carver): add tooling to help carve out sub-modules (#4417)

internal/carver/README.md

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+# carver
+
+This is a tool used to carve out new modules in cloud.google.com/go.
+
+## Usage
+
+```bash
+go run cmd/main.go \
+  -parent=/path/to/google-cloud-go \
+  -child=asset \
+  -repo-metadata=/path/to/google-cloud-go/internal/.repo-metadata-full.json
+```

internal/carver/cmd/_CHANGES.md.txt

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+# Changes
+
+## v0.1.0
+
+This is the first tag to carve out {{.Package}} as its own module. See
+[Add a module to a multi-module repository](https://github.com/golang/go/wiki/Modules#is-it-possible-to-add-a-module-to-a-multi-module-repository).

internal/carver/cmd/_README.md.txt

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+# {{.Name}}
+
+[![Go Reference](https://pkg.go.dev/badge/{{.ImportPath}}.svg)](https://pkg.go.dev/{{.ImportPath}})
+
+Go Client Library for {{.Name}}.
+
+## Install
+
+```bash
+go get {{.ImportPath}}
+```
+
+## Stability
+
+The stability of this module is indicated by SemVer.
+
+However, a `v1+` module may have breaking changes in two scenarios:
+
+* Packages with `alpha` or `beta` in the import path
+* The GoDoc has an explicit stability disclaimer (for example, for an experimental feature).
+
+## Go Version Support
+
+See the [Go Versions Supported](https://github.com/googleapis/google-cloud-go#go-versions-supported)
+section in the root directory's README.
+
+## Authorization
+
+See the [Authorization](https://github.com/googleapis/google-cloud-go#authorization)
+section in the root directory's README.
+
+## Contributing
+
+Contributions are welcome. Please, see the [CONTRIBUTING](https://github.com/GoogleCloudPlatform/google-cloud-go/blob/master/CONTRIBUTING.md)
+document for details.
+
+Please note that this project is released with a Contributor Code of Conduct.
+By participating in this project you agree to abide by its terms. See
+[Contributor Code of Conduct](https://github.com/GoogleCloudPlatform/google-cloud-go/blob/master/CONTRIBUTING.md#contributor-code-of-conduct)
+for more information.

internal/carver/cmd/_tidyhack_tmpl.txt

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+// Copyright {{.Year}} Google LLC
+//
+// 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.
+
+// This file, and the {{.RootMod}} import, won't actually become part of
+// the resultant binary.
+// +build modhack
+
+package {{.Package}}
+
+// Necessary for safely adding multi-module repo. See: https://github.com/golang/go/wiki/Modules#is-it-possible-to-add-a-module-to-a-multi-module-repository
+import _ "{{.RootMod}}"