Ignore non-jar files when creating resource loaders

[peterphan]

There can be cases where a non-jar file is added to the classpath / java.class.path system property. See https://github.com/JetBrains/gradle-intellij-plugin/issues/1009 This diff ignores non-jar files when loading the classpath, similar to https://github.com/autonomousapps/dependency-analysis-android-gradle-plugin/commit/9dcc6e22bcec9264cb5f06d5299e9df5fd8446fc