Accept optional dependencies as features

[heksesang]

Currently it doesn't seem to accept optional dependencies as features, like poem-openapi = { version = "*", features = ["url"] } or tracing = { version ="*", features = ["log"] }.

I assume this is related to crates.io not listing them as features (see https://crates.io/api/v1/crates/poem-openapi/5.1.2), so the solution would likely be to also parse the list of dependendencies (see https://crates.io/api/v1/crates/poem-openapi/5.1.2/dependencies) and add the optional ones which are not used as dep:... anywhere to the list of accepted features.