Before 5b7cee6221 JVM CLI compiler
was calling `KotlinToJVMBytecodeCompiler.compileBunchOfSources`.
`compileBunchOfSources` detected possible main classes,
and filled the Main-Class attribute in output jar
if if there was only one candidate.
After the change JVM CLI began calling
`KotlinToJVMBytecodeCompiler.compileModules`, which was not searching for a main class.
This change adds searching for main classes to `compileModules`.
We search for a main class only when one module is compiled,
and an output is written a jar file (so the change only affects JVM CLI compilation).
#KT-32272 Fixed
Alexey Tsvetkov
ffa191d91d