Support external $ref

progenitor panicks if the input doc includes references to external files

e.g. input

components:
  schemas:
    CartesianCoordinate:
      $ref: "./schemas/CartesianCoordinate.yaml"

result

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/jayvdb/rust/typify/typify-impl/src/convert.rs:772:47
stack backtrace:
   0:     0x55d23fd6d64d - std::backtrace_rs::backtrace::libunwind::trace::h9135f25bc195152c
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x55d23fd6d64d - std::backtrace_rs::backtrace::trace_unsynchronized::h015ee85be510df51
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x55d23fd6d64d - std::sys_common::backtrace::_print_fmt::h5fad03caa9652a2c
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x55d23fd6d64d - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h2b42ca28d244e5c7
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x55d23fd93d6c - core::fmt::write::h401e827d053130ed
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/fmt/mod.rs:1198:17
   5:     0x55d23fd6a301 - std::io::Write::write_fmt::hffec93268f5cde32
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/io/mod.rs:1672:15
   6:     0x55d23fd6ee15 - std::sys_common::backtrace::_print::h180c4c706ee1d3fb
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x55d23fd6ee15 - std::sys_common::backtrace::print::hd0c35d18765761c9
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x55d23fd6ee15 - std::panicking::default_hook::{{closure}}::h1f023310983bc730
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:295:22
   9:     0x55d23fd6eb31 - std::panicking::default_hook::h188fec3334afd5be
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:314:9
  10:     0x55d23fd6f3a6 - std::panicking::rust_panic_with_hook::hf26e9d4f97b40096
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:698:17
  11:     0x55d23fd6f259 - std::panicking::begin_panic_handler::{{closure}}::hfab912107608087a
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:586:13
  12:     0x55d23fd6db44 - std::sys_common::backtrace::__rust_end_short_backtrace::h434b685ce8d9965b
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/sys_common/backtrace.rs:138:18
  13:     0x55d23fd6efc9 - rust_begin_unwind
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:584:5
  14:     0x55d23f8c6683 - core::panicking::panic_fmt::ha6dc7f2ab2479463
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/panicking.rs:142:14
  15:     0x55d23f8c654d - core::panicking::panic::hb3ad04c589a0e3c8
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/panicking.rs:48:5
  16:     0x55d23faf5ac4 - typify_impl::convert::<impl typify_impl::TypeSpace>::convert_reference::h38b314185e35cd4d
  17:     0x55d23faf2f52 - typify_impl::convert::<impl typify_impl::TypeSpace>::convert_schema_object::hed6926d88a31c8a9
  18:     0x55d23faf26a6 - typify_impl::convert::<impl typify_impl::TypeSpace>::convert_schema::h4a1033e80f5a0227
  19:     0x55d23fa4bfe2 - typify_impl::TypeSpace::add_ref_types::h6da788964b09931e
  20:     0x55d23fa2dc56 - progenitor_impl::Generator::generate_tokens::h9a98679782fb4367
  21:     0x55d23fa31f78 - progenitor_impl::Generator::generate_text_impl::h52fba84e175381f2
  22:     0x55d23fa31f35 - progenitor_impl::Generator::generate_text::hf4b2309eba436570
  23:     0x55d23f95f229 - progenitor::main::hd3c6b58b88574d4b
  24:     0x55d23f9ecaf3 - std::sys_common::backtrace::__rust_begin_short_backtrace::h0594b750e9743dbe
  25:     0x55d23f9ecd79 - std::rt::lang_start::{{closure}}::h55bb4683229af303
  26:     0x55d23fd651ae - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::hcdfee62722e5e4b8
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/ops/function.rs:280:13
  27:     0x55d23fd651ae - std::panicking::try::do_call::h84ca51609826746f
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:492:40
  28:     0x55d23fd651ae - std::panicking::try::hd58075e533b8e0cb
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:456:19
  29:     0x55d23fd651ae - std::panic::catch_unwind::h1ebac24d83cb6ce2
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panic.rs:137:14
  30:     0x55d23fd651ae - std::rt::lang_start_internal::{{closure}}::h0145388a1edd1640
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/rt.rs:128:48
  31:     0x55d23fd651ae - std::panicking::try::do_call::h7630182e534a0a32
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:492:40
  32:     0x55d23fd651ae - std::panicking::try::h05b6544f0c6331dc
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:456:19
  33:     0x55d23fd651ae - std::panic::catch_unwind::h77b2ba8fd3309f34
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panic.rs:137:14
  34:     0x55d23fd651ae - std::rt::lang_start_internal::h6612c8a7a6861b8b
                               at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/rt.rs:128:20
  35:     0x55d23f9635e2 - main
  36:     0x7f9b51ebf5b0 - __libc_start_call_main
  37:     0x7f9b51ebf679 - __libc_start_main@GLIBC_2.2.5
  38:     0x55d23f8c6935 - _start
                               at /home/abuild/rpmbuild/BUILD/glibc-2.36/csu/../sysdeps/x86_64/start.S:115
  39:                0x0 - <unknown>

Start in typify-impl/src/convert.rs at convert_reference, around the reported line 772, and reproduce the panic with the YAML document and external schema reference shown here. Trace how the input document resolves references; done means generation handles the external file reference without panicking.