Migrate jib-{maven|gradle}-plugin to use published extension API

[chanseokoh]

Migrate all projects to use published artifacts. I missed jib-core in #2443, so fixing it here too.

I have checked the contents of all 4 artifacts, and there's no class duplication in any published artifact.

[chanseokoh]

Ugh, the published Gradle API is the older version without the custom extension properties. I need to republish the API.

[chanseokoh]

$ ./gradlew buildEnv with the plugin built at HEAD (2.2.1-SNAPSHOT) shows the following classpath graph.

classpath
...
\--- com.google.cloud.tools.jib:com.google.cloud.tools.jib.gradle.plugin:2.2.1-SNAPSHOT
     \--- com.google.cloud.tools:jib-gradle-plugin:2.2.1-SNAPSHOT
          +--- com.google.cloud.tools:jib-build-plan:0.1.0

          ...

          +--- com.google.cloud.tools:jib-plugins-extension-common:0.1.0
          \--- com.google.cloud.tools:jib-gradle-plugin-extension-api:0.1.1
               +--- com.google.cloud.tools:jib-build-plan:0.1.0
               \--- com.google.cloud.tools:jib-plugins-extension-common:0.1.0

oops

[chanseokoh]

This works, and nothing major breaks if we merge this. However, I'd like to complete migration after publishing new API being updated by #2464.

[chanseokoh]

Ready for review! Updated the versions after publishing new artifacts. Although jib-gradle-plugin-extension-api currently depends on jib-plugins-extension-common:0.1.0, we should be good at runtime as 0.2.0 will be used.

          +--- com.google.cloud.tools:jib-plugins-extension-common:0.2.0
          \--- com.google.cloud.tools:jib-gradle-plugin-extension-api:0.2.0
               +--- com.google.cloud.tools:jib-build-plan:0.1.0
               \--- com.google.cloud.tools:jib-plugins-extension-common:0.1.0 -> 0.2.0

If a Travis build fails, it would be due to the Google Maven Central mirror running behind syncing.