[kernel] Allow plugins to register importers with the same name

src/kernel_internals/parsing/logic_lexer.mll

src/kernel_internals/parsing/logic_parser.mly

src/kernel_services/ast_data/cil_types.ml

src/kernel_services/ast_printing/cil_printer.ml

src/kernel_services/ast_printing/cil_types_debug.ml

src/kernel_services/ast_printing/logic_print.ml

src/kernel_services/ast_queries/acsl_extension.ml

src/kernel_services/ast_queries/acsl_extension.mli

src/kernel_services/ast_queries/logic_env.ml

src/kernel_services/ast_queries/logic_env.mli

src/kernel_services/ast_queries/logic_typing.ml

src/kernel_services/ast_queries/logic_typing.mli

src/kernel_services/parsetree/logic_ptree.ml