Do not use File.isHidden() as it is platform-specific

[abstratt]

At a few places, such as this, the plug-in uses java.io.File.isHidden() to decide whether to ignore a file.

However, the behavior of File.isHidden() is platform-dependant. On Unixes, it considers files starting with "." to be hidden. On Windows, only files that actually have the "hidden" attribute set are considered hidden.

That produces results that are different depending on the platform. For instance:

mvn package -DonlyMuleSources=true

will result in a JAR file that contains any files starting with "." on Windows, but not on Linux/MacOS.

A workaround could be to consider a file hidden if it either starts with "." or File.isHidden().