Use `dep:` prefix in feature dependencies

[Dopplerian]

This prevents users from manually enabling optional dependencies as if they were features.

Without this, the following Cargo.toml would be valid:

[package]
name = "some-package"
version = "0.1.0"

[dependencies]
fang = { version = "0.10.4" , default-features = false, features = ["bb8-postgres"]}

The Cargo Book recomends this.