:icon_cry: How the JDK tool could know the (directory that contained the first element of the) Fully Qualified Package-Name of the required class that yet to be searched ?

Your question isn't very clear; when you say JDK tool, which tool are you referring to? Also, if you are talking of "java", it doesn't know the "fully qualified class name", you have to specify it. When you write:

java -cp . net.sanjayts.Main

the executable searches for the directory structure net/sanjayts in the classpath and executes the bytecode present in the net/sanjayts/Main.class file (if it exists).