Preserve optional dependencies when creating features in the expanded crate

[antalsz]

I found that when I had an optional dependency in a crate and a feature that depended on it with the same name, the expanded crate would not create explicit dependencies on the dep:foo optional features, and then Cargo would complain that dep:foo was unused. For instance:

# Original my_crate Cargo.toml

[features]
foo = ["dep:foo", "other/foo"]

[dependencies]
foo = { version = "1.0.0", optional = true }

would give rise to

error: failed to parse manifest at `/.../macrotest000/Cargo.toml`

Caused by:
  optional dependency `foo` is not included in any feature
  Make sure that `dep:foo` is included in one of features in the [features] table.

because the generated Cargo.toml would look like

# macrotest generated Cargo.toml

[features]

foo = ["original_crate/foo"]

[dependencies]
foo = { version = "1.0.0", optional = true }

and the automatic foo feature would not be created. This PR preserves all dep:… features when creating the expanded Cargo.toml:

# New macrotest generated Cargo.toml

[features]

foo = ["original_crate/foo", "dep:foo"]

[dependencies]
foo = { version = "1.0.0", optional = true }

[taiki-e]

Published in v1.0.13.