(hugr-model) Model round-trip panics with `AliasDecl`

[cqc-alec]

With the HUGR constructed in this test, serialization to model followed by deserialization panics with:

thread 'builder::module::test::simple_alias' (1243268) panicked at hugr-core/src/envelope.rs:301:63:
called `Result::unwrap()` on an `Err` value: Payload { source: PayloadError(ModelBinary(Import(ImportError { inner: Context(Context(Context(Context(Context(Invalid("`qubit_state` is not a valid symbol name"), "term 3 as `Type`"), "function inputs"), "function type"), "symbol `_2` defined by node 1"), "`define-func` node with id 1") }))), partial_description: PackageDesc { header: EnvelopeHeader { format: ModelWithExtensions, zstd: true }, modules: [Some(ModuleDesc { num_nodes: Some(5), entrypoint: None, used_extensions_resolved: None, generator: None, used_extensions_generator: None, public_symbols: None })], packaged_extensions: [] } }

[cqc-alec]

The issue is that when we try to import the DefineFunc node that uses the alias to express one of its types, we just try to interpret the alias name as a type and fail.

Fixing this is low priority as we will probably be getting rid of aliases anyway (#2558 ).