Fix feature ref diagnostic to include plain dep enablements

[CathalMullan]

Resolves #354

The "used in feature" diagnostic now includes plain dep enablements, like "foo" instead of "dep:foo".

shear/unused_optional_dependency

  ⚠ unused optional dependency `smol`
    ╭─[sqlx-macros-core/Cargo.toml:64:1]
 63 │ async-std = { workspace = true, optional = true }
 64 │ smol = { workspace = true, optional = true }
    · ──┬─
    ·   ╰── not used in code
 65 │ tokio = { workspace = true, optional = true }
    ╰────

Advice:
  ☞ removing an optional dependency may be a breaking change

Advice:
  ☞ used in feature `_rt-smol`
    ╭─[sqlx-macros-core/Cargo.toml:17:13]
 16 │ _rt-async-std = ["async-std", "sqlx-core/_rt-async-std"]
 17 │ _rt-smol = ["smol", "sqlx-core/_rt-smol"]
    ·             ───┬──
    ·                ╰── enabled here
 18 │ _rt-tokio = ["tokio", "sqlx-core/_rt-tokio"]
    ╰────

[Boshen]

Nice!