cmd/go/internal/tool: include dynamically buildable tools

[Agent-Hellboy]

go tool only listed binaries already built into pkg/tool, so users
couldn't discover builtin tools that can be built on demand. Extend the
listing to include cmd/ tools that can be built with the current
toolchain while avoiding duplicates and keeping the output sorted.

Fixes #75960

[gopherbot]

This PR (HEAD: 5a39834) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/714560.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/714560.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Michael Matloob:

Patch Set 1:

(2 comments)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/714560.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Prince Roshan:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/714560.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: 0784336) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/714560.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Prince Roshan:

Patch Set 3:

(2 comments)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/714560.
After addressing review feedback, remember to publish your drafts!