Use reflection to get user's main method, not direct link from the importing

[aaaaaa2493]

Relates to #46

[vyahhi]

In Stepik we use the following bash script to get the main class name in Java (and somewhat similar in Scala and Kotlin):

#!/bin/bash
# Grep *.class files in the given directory to find one with psvm method defined
# Usage: java_lookup_main.sh DIRECTORY_PATH

PSVM_REGEX="public static( final)? void main\(java\.lang\.String(\[\]|\.\.\.)\)"

shopt -s nullglob
for f in "$1"/*.class; do
    if javap -p "$f" 2> /dev/null | grep -Pq "$PSVM_REGEX"; then
        filename=$(basename "$f")
        echo -n "${filename%.*}"
        exit 0
    fi
done
exit 1

It's here https://github.com/StepicOrg/epicbox-images/blob/145de3624e81b358406dffdb39c485bbc2e16144/epicbox-java/11/java_lookup_main.sh.

Full code to work with this script: https://github.com/bioinf/edy/blob/e7fda90797557eecf841b86ef78e6ebc71a5572b/plugins/stepic_plugins/quizzes/code/runners.py#L192..L242

[aaaaaa2493]

Also should be checked if there are more than one Main method and show feedback to the user that there should be just one main method

[aaaaaa2493]

Fixed in 55e1caf4f37783a49ab8d378b0b7a8820fbbfb2a

The script will be useful for implementing #91