Exempt 3 enums from the large_enum_variant linter

Exempt 3 enums from the large_enum_variant linter

Exempt as opposed to box, like suggested:

    warning: large size difference between variants
      --> src/cargo/core/manifest.rs:21:5
       |
    21 |     Real(Manifest),
       |     ^^^^^^^^^^^^^^
       |
       = note: #[warn(large_enum_variant)] on by default
       = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#large_enum_variant
    help: consider boxing the large fields to reduce the total size of the enum
       |
    21 |     Real(Box<Manifest>),
       |          ^^^^^^^^^^^^^

    warning: large size difference between variants
      --> src/cargo/core/workspace.rs:91:5
       |
    91 |     Package(Package),
       |     ^^^^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#large_enum_variant
    help: consider boxing the large fields to reduce the total size of the enum
       |
    91 |     Package(Box<Package>),
       |             ^^^^^^^^^^^^

    warning: large size difference between variants
       --> src/cargo/util/toml/mod.rs:156:5
        |
    156 |     Detailed(DetailedTomlDependency),
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#large_enum_variant
    help: consider boxing the large fields to reduce the total size of the enum
        |
    156 |     Detailed(Box<DetailedTomlDependency>),
        |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^