Add heuristics to speed up Jib check in skaffold init

[TadCordle]

With this PR, skaffold first checks for Jib-related strings in the build file before attempting to run the task/goal, rather than attempting to call into Jib any time a build.gradle or pom.xml is found. This eliminates a lot of false positives and speeds up skaffold init considerably on projects that don't have Jib configured.

Here are the timings for running skaffold init --XXenableJibInit --analyze on a 2-module maven project with no Jib configurations, before and after this PR:

• Before: 5.593s
• After: 0.246s

Timings before and after are about the same if Jib is configured on all modules. This PR seems to save around 2-3 seconds for every module Jib is not configured.

It still may be a good idea to consider automatically applying Jib on Java projects where it isn't already configured, but in the meantime this seems like a good improvement to have.

Release Notes

Speed up Jib checks in skaffold init on Java projects.

[codecov]

Codecov Report

Merging #3120 into master will increase coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/jib/jib_init.go	89.55% <100%> (+0.66%)	⬆️
pkg/skaffold/server/server.go	58.57% <0%> (ø)	⬆️

[chanseokoh]

This is really cool. It could be worse in the status quo when artifacts are not downloaded in Maven/Gradle caches.

The only corner case I have been thinking about is the multi-module/project case, where I can imagine <artifactId>jib-maven-plugin</artifactId> is defined only in the parent project or the other way around. Will skaffold init work for both Maven and Gradle in these cases?

[TadCordle]

Assuming I tested everything correctly, my findings:

Maven

• Parent only: jib:_skaffold-init runs on the root project and lists all modules.
• Module only: Parent pom is skipped, and jib:_skaffold-init runs on the module directly.

Gradle

• Parent only: If jib isn't applied to the subprojects, _jibSkaffoldInit won't list them.
• Module only: Parent is skipped and _jibSkaffoldInit runs on the module directly.

[chanseokoh]

Are your findings before or after this quick check? Doesn't this PR change any behavior? I think I better talk to you offline.

[loosebazooka]

Does this work if jib is configured further down the tree in a multimodule project? (and not at the root)