add a warning if a build.yml file is found in the doctor command

[jakemac53]

Fixes #3615

Note that this would only give the warning if running the doctor command, which people may not be in the habit of running, but is typically where we put these kinds of diagnostics.

@schaipipat out of curiosity were you aware of the doctor command? Would this have helped you or not?

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:build_runner	2.4.7	ready to publish	build_runner-v2.4.7
package:build	2.4.2-wip	WIP (no publish necessary)
package:build_config	1.1.2-wip	WIP (no publish necessary)
package:scratch_space	1.0.3-wip	WIP (no publish necessary)
package:build_test	2.2.2-wip	WIP (no publish necessary)
package:build_resolvers	2.4.1	already published at pub.dev
package:build_web_compilers	4.0.7	already published at pub.dev
package:build_runner_core	7.2.12-wip	WIP (no publish necessary)
package:build_daemon	4.0.1	already published at pub.dev
package:build_modules	5.0.5	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[natebosch]

This LGTM. We could also consider a verbose log like "No build.yaml configuration file, using defaults." and a different verbose log for when the file is read. Anyone running with -v could see this without having to run a separate doctor command.

[jakemac53]

I tried originally putting this into the code that actually looks for these files, but it ended up getting complicated because those APIs are actually called for all transitive packages deps, and don't have access to a logging API. So this was the simplest approach.

[schaipipat]

@jakemac53 Not really. All the tools that using build_runner never mention about doctor command so I haven't run it when I start facing the issue.

[jakemac53]

I am going to go ahead and land this because it is better than not having anything - longer term I think we need to figure out how to promote the doctor command more I think.