Implement namespace-specified imports and fix ExplicitNamespaces

The GHC Steering Committee has accepted proposal 581: namespace-specified imports. This tightens up the specification of ExplicitNamespaces, allows data as a namespace specifier instead of pattern, and permits imports to use type .. and data .. to refer to all items in the corresponding namespaces.

• #12488 "Explicit namespaces doesn't enforce namespaces" (bug)
• #22581 "Explicit namespace type import can bring a value into scope" (bug)
• #25983 (closed) "Poor warning message with -Wdodgy-imports and non-existent subordinate items" (bug)
• #25984 (closed) "Dodgy import breaks a hiding clause" (bug)
• #25991 (closed) "Punned class method and associated type break explicit import" (bug)
• #25899 "Allow the data keyword in import and export lists"
• #25900 "Deprecate the pattern keyword in import and export lists"
• #25901 "Wildcards .. in import and export lists"