fix(stageleft_tool): include strong-ref-enabled and target-specific deps in __deps

[shadaj]

Completes the fix for #62. PR #65 made optional
dependencies show up in __deps behind #[cfg(any(feature = ...))]
gates, but three gaps remained (the first two are hit by real manifests
such as hydro_lang's):

1. Strong pkg/feature references were not treated as enabling an
   optional dependency. Cargo's feat = ["tokio-stream/sync"] enables
   the optional dep tokio-stream, but optional_dep_features only
   matched dep:<name> refs, so the re-export was cfg'd out when the
   dep was enabled solely via a strong ref. The old
   dep:<name>/... prefix check was dead code (not valid Cargo syntax)
   and has been removed. Weak pkg?/feature refs correctly do not
   count as gating. The implicit feature is now appended only when no
   dep:<name> reference exists anywhere (matching Cargo semantics).

2. [target.'cfg(...)'.dependencies] tables were ignored entirely, so
   target-specific deps (e.g. hydro_lang's linux-only procfs) never
   appeared in __deps. They are now collected and gated on the
   target's cfg predicate, composed with feature gates for optional
   deps: #[cfg(all(<target cfg>, any(feature = ...)))]. A dep
   declared in multiple tables is re-exported once, gated by any()
   of its per-declaration predicates, or ungated if any declaration is
   unconditional. Non-cfg(...) target keys (plain target triples)
   are skipped with a cargo::warning since they cannot be expressed
   as a cfg gate.

3. A missing [dependencies] table no longer panics.

Implementation: gen_deps_module now builds a list of Dep entries
(name, package alias, Vec<DepDeclaration>) collected from the main
and target dependency tables via collect_deps_from_table, and
cfg_attr_for_declarations derives the combined #[cfg(...)]
attribute used for both the pub use re-exports and the runtime
add_deps_reexport registrations.

Tests:

• 7 new unit tests in stageleft_tool covering strong refs, weak refs,
  target-specific deps (plain and optional), deps declared in both the
  main and target tables, deps in multiple target tables, and a
  manifest with no [dependencies] table.
• stageleft_test_no_entry now declares my_strong_ref_feature
  (strong once_cell/std ref), my_weak_ref_feature (weak
  once_cell?/std ref), and a [target.'cfg(unix)'.dependencies]
  libc dep; the codegen snapshot shows the new gating, e.g.
  #[cfg(any(feature = "my_strong_ref_feature", feature = "once_cell"))]
  and #[cfg(unix)] pub use libc;.

Note: the snapshot must be regenerated with a full workspace build
(cargo test --workspace --all-targets, as CI does) — building the
crate alone changes prettyplease's output formatting due to feature
unification of syn's full feature.

Verified with cargo test --workspace --all-targets (61 tests pass),
cargo fmt --all -- --check, and
cargo clippy --all-targets -- -D warnings.