Simplify the tooling bash scripts

[stuartmorgan]

• Rename incremental_build.sh to tool_runner.sh since it has been used for things other than building for a while, and also isn't always incremental.
• Replace check_publish.sh with tool_runner.sh to reduce the number of special-case scripts. This is a small behavioral change in that now PRs that don't change any packages will test all packages, as with our other scripts.
• Eliminate check_changed_packages. All critical uses have now been replaced by the Dart version of the logic, and the one remaining use was purely for an error message that says something that should be relatively easy to figure out from context anyway.

This means we have less bash code to maintain (without unit tests) and understand.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[cyanglaz]

LGTM