Could not compile some features without default features turned on

[vjtch]

Describe the bug
When selecting only _deflate-any or deflate-flate2 feature with default-features turned off it's not possible to compile zip crate. Both features are missing some flate2 dependencies.

To Reproduce
In Cargo.toml use this dependency settings

zip = { version = "2.1.6", default-features = false, features = ["_deflate-any"] }

Expected behavior
Feature should enable all necessary components to successfully compile or at least provide compilation error with message which features user should add.

Temporary solution
In Cargo.toml use this dependency settings (with added deflate feature)

zip = { version = "2.1.6", default-features = false, features = ["_deflate-any", "deflate"] }

Possible fix
For _deflate-any add deflate-flate2 and for deflate-flate2 add flate2/rust_backend (produces unused import warning).

[features]
_deflate-any = ["deflate-flate2"]
deflate-flate2 = ["flate2/any_impl", "flate2/rust_backend", "_deflate-any"]

Produced errors (_deflate-any, for deflate-flate2 are different)

error[E0433]: failed to resolve: use of undeclared crate or module `flate2`
   --> /zip2/src/compression.rs:195:14
    |
195 |     Deflated(flate2::bufread::DeflateDecoder<R>),
    |              ^^^^^^ use of undeclared crate or module `flate2`

error[E0433]: failed to resolve: use of undeclared crate or module `flate2`
   --> /zip2/src/compression.rs:234:40
    |
234 |                 Decompressor::Deflated(flate2::bufread::DeflateDecoder::new(reader))
    |                                        ^^^^^^ use of undeclared crate or module `flate2`

warning: unused import: `core::num::NonZeroU64`
  --> /zip2/src/write.rs:19:5
   |
19 | use core::num::NonZeroU64;
   |     ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

error[E0433]: failed to resolve: use of undeclared type `Compression`
    --> /zip2/src/write.rs:1597:25
     |
1597 |                         Compression::default().level() as i64
     |                         ^^^^^^^^^^^ use of undeclared type `Compression`

error[E0308]: mismatched types
    --> /zip2/src/write.rs:1590:48
     |
1590 |                   CompressionMethod::Deflated => {
     |  ________________________________________________^
1591 | |                     let default = if cfg!(all(
1592 | |                         feature = "deflate-zopfli",
1593 | |                         not(feature = "deflate-flate2")
...    |
1644 | |                     }
1645 | |                 }
     | |_________________^ expected `Result<Box<...>, ...>`, found `()`
     |
     = note:   expected enum `Result<Box<(dyn FnOnce(MaybeEncrypted<W>) -> write::GenericZipWriter<W> + 'static)>, ZipError>`
             found unit type `()`

error[E0433]: failed to resolve: use of undeclared type `Compression`
    --> /zip2/src/write.rs:1761:9
     |
1761 |         Compression::fast().level() as i64
     |         ^^^^^^^^^^^ use of undeclared type `Compression`

error[E0433]: failed to resolve: use of undeclared type `Compression`
    --> /zip2/src/write.rs:1763:9
     |
1763 |         Compression::best().level() as i64 + 1
     |         ^^^^^^^^^^^ use of undeclared type `Compression`

error[E0433]: failed to resolve: use of undeclared type `Compression`
    --> /zip2/src/write.rs:1766:15
     |
1766 |     let max = Compression::best().level() as i64
     |               ^^^^^^^^^^^ use of undeclared type `Compression`

[CommanderStorm]

features starting with _ are private features. They are not to be relied on by external users.
See the relevant cargo issue for the discussion rust-lang/cargo#10882

They are not listed on docs.rs like f.ex. here: https://docs.rs/crate/zip/2.1.6/features

[Pr0methean]

As well, README.md states (emphasis added):

• deflate-flate2: Combine this with any flate2 feature flag that enables a back-end, to support deflate compression at quality 1..=9.