Allow buildutil.TagsFlag to handle comma-delimited tag lists

[yaaase]

As golang has deprecated space-delimited build tags in favor of comma-delimited, this PR allows buildutil.TagsFlag to handle comma-delimited build tags

Fixes golang/go#44787

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[yaaase]

@googlebot I fixed it.

[gopherbot]

This PR (HEAD: fe39683) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/tools/+/284214 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://golang.org/doc/contribute.html#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/284214.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Bryan C. Mills:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/284214.
After addressing review feedback, remember to publish your drafts!

[yaaase]

Issue created: golang/go#44787

[gopherbot]

Message from Alberto Donizetti:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/284214.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Bryan C. Mills:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/284214.
After addressing review feedback, remember to publish your drafts!

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[gopherbot]

This PR (HEAD: 559d77b) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/tools/+/284214 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info