fix: don't lint private-type-ref for pure type alias re-exports

[crowlbot]

Problem

deno doc --lint reports a spurious private-type-ref diagnostic for a type
alias that simply re-names another type:

interface DumperStateOptions {}

export type DumpOptions = DumperStateOptions;

Type 'DumpOptions' references type 'DumperStateOptions' which is not exported
from a root module.

A type alias whose body is just another type reference (export type A = B) is
effectively another name for the referenced type — B is re-exposed through the
alias and should be considered part of the public API, not a private type leaked
into it.

Fix

When collecting the dependencies of an exported symbol, detect declarations that
are pure type aliases — a type X = Y whose body is a bare TsTypeRef with no
type arguments — and flag their references as alias re-exports
(SymbolDeclDeps::is_alias_reexport). The diagnostics collector then skips
emitting private-type-ref for those references.

The referenced type is still included in the generated documentation (it remains
a non-exported public type), so only the spurious lint diagnostic is suppressed.

Tests

• New spec tests/specs/private_type_via_type_alias.txt pins the reproduction
  from the issue: no diagnostic, with the referenced interface still documented.
• Updated tests/specs/reexport_deep.txt, which exercises the same
  export type A = B pattern across re-exported modules — its private-type-ref
  diagnostics are now correctly suppressed (the deep re-export resolution it
  primarily tests is unchanged).

Fixes #429

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