SYSML2_ — Update language description and concrete syntax related to imports

The resolutions by the KerML FTF to the following issues make restrictions to the functionality of imports in KerML and hence, by reference, in SysML, also:

• KERML_-73 Disallow public imports at root level
• KERML_-74 Make imports private by default
• KERML_-75 Restrict the functionality of recursive import

The discussion of imports in Clause 7 Language Description need to be updated to reflect these changes (particularly in 7.5.3 Imports). In addition, adopting the requirement to always show visibility of imports in the concrete syntax, consistent with KERML_-74, requires changes in

• 8.2.2.5.1 Packages (textual notation)
• 8.2.3.5 Namespaces and Packages Graphical Notation
• 8.3.5 Namespaces and Packages Abstract Syntax, Figure 5 Namespaces