make computePackageNameErrors more accurate

[kzvi]

implements these aspects of what constitutes a valid Java package name:

• package names can't include java reserved words as components
• no component of a package name can start with a number
• any component of a package name can start with an underscore
• a package name is allowed to have only one component
• no component of a package name may consist solely of an underscore

i made this change because i was having issues with the template generator not allowing legal package names (the added strictness about reserved words is not really necessary i guess but it can't hurt)