[Kernel] add Filepath.is_known and refactor to use it

src/kernel_internals/runtime/special_hooks.ml

src/kernel_services/ast_queries/json_compilation_database.ml

src/kernel_services/cmdline_parameters/parameter_builder.ml

src/kernel_services/cmdline_parameters/parameter_sig.mli

src/plugins/aorai/aorai_option.ml

src/plugins/aorai/aorai_register.ml

src/plugins/callgraph/register.ml

src/plugins/markdown-report/md_gen.ml

src/plugins/markdown-report/sarif_gen.ml

src/plugins/metrics/metrics_acsl.ml

src/plugins/metrics/metrics_cilast.ml

src/plugins/report/classify.ml

src/plugins/report/csv.ml

src/plugins/server/server_doc.ml

src/plugins/value/domains/numerors/numerors_domain.ml

src/plugins/value/utils/red_statuses.ml

src/plugins/value/utils/value_perf.ml