Tasks labels: make them consistent

In #679 (closed) I'm trying to improve the task labels because we have the subject (often the package name) available in Task.dynamic_data.subject.

Before #679 (closed) we had:

• Sbuild: "build a package"
• Lintian: "lintian"
• Autopkgtset: "autopkgtest"

During #679 (closed):

• Sbuild: "build {package_name}" or "build a package" (if package name is not known)
• Lintian: "lintian {package_name}" or "lintian" (if package name is not known)
• Autopkgtest: "autopkgtest {package_name}" or "autopkgtest (if package name is now known)

I thought of consolidating them if possible. @cjwatson said (!1581 (comment 575402)):

We should probably decide whether we want task labels to be short English phrases, or to be more abbreviated. If the former, then this should be something like f"run autopkgtests for {self.dynamic_data.subject}". But maybe the latter is better, since we do render these in tables and it would be easy for them to get too long by accident.

How about applying a consistent convention of "task name: subject"? I think that would look better and clearer in a table of running work requests or similar.

One other thing that occurs to me here is that the information we want to show in the task label isn't quite the same as the information we want to record in task statistics. For task statistics, the subject deliberately doesn't include version information, because we want to aggregate statistics across versions. But for the task label, it would be much more useful to show something like "autopkgtest: hello/1.0-1/amd64".

I thought that this needs a wider discussion outside the MR, and that maybe others (@enrico / @hertzog / @stefanor ) might have something to comment.

Progress:

• AssembleSignedSource
• Autopkgtest
• Blhc
• DebDiff
• ExtractForSigning
• Lintian
• MakeSourcePackageUpload
• MergeUploads
• MmDebstrap
• Noop
• Piuparts
• Sbuild
• SimpleSystemImageBuild
• SystemBootstrap
• SystemImageBuild