rework main class finding

[chanseokoh]

Fixes #2456.

The logic has changed:

1. main class explicitly configured (via jib.container.mainClass) but invalid

   • before: attempted to infer a main class under src/.
     • problems and weirdness:
       • never looked into mainClassFromJar despite checking src/ (inconsistency)
       • configured jib.container.mainClass ignored if a valid main is found under `src/', no warning at all
   • after: error

2. main class not configured, invalid mainClassFromJar, and no or multiple mains found under src/ (i.e., the only main class we could ever get is from the jar-plugin)

   • before: warned mainClassFromJar is invalid, but proceeded with it (this mainClassFromJar used to be called "backup" in our tests)
   • after: error

So, the case in #2456 falls into 2) with multiple mains, and now Jib will fail while printing

[WARNING] 'mainClass' configured in 'maven-jar-plugin' is not a valid Java class: ${start-class}

and

[ERROR] ... Multiple valid main classes were found: hello.Application, hello.Foo, perhaps you should add a 'mainClass' configuration to jib-maven-plugin

As usual, if only one (valid) main is found under src/ while mainClassFromJar is invalid, Jib will continue to use the inferred one rather than mainClassFromJar.

[chanseokoh]

Note this is not a new method; nothing changed.

[chanseokoh]

Oh, forgot CHANGELOG. Will add.

[chanseokoh]

Merging. Feel free to update CHANGELOG afterwards if that doesn't read natural.