…eps in `__deps`

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`.

Co-authored-by: Infinity 🤖 <infinity@hydro.run>
PR: #89