If a user goes through the wizard New Java Project with Existing Sources, he is
able to choose a Source Package Folder which is not a root directory of user's
Java sources.

If the user for example chooses a subfolder of his src/ folder, hoping that he
will get a project with Java sources from only that Java package, instead he
gets broken project with invalid package paths.

In this case, the user should be warned in the wizard that the Source Package
Folder he has chosen is not correct and that it must be a root folder where the
package folders begin, not in the middle of the path.

This warning will prevent confusion and help avoid the cases of broken project
sources.