Allow #[serde(serde_path = "...")] to override extern crate serde

This is intended to be used by other crates which provide their own proc
macros and use serde internally. Today there's no consistent way to put
`#[derive(Deserialize)]` on a struct that consistently works, since
crates may be using either `features = ["derive"]` or relying on
`serde_derive` separately.

Even if we assume that everyone is using `features = ["derive"]`,
without this commit, any crate which generates
`#[derive(serde::Deserialize)]` forces its consumers to put `serde` in
their `Cargo.toml`, even if they aren't otherwise using serde for
anything.

Examples of crates which suffer from this in the real world are
tower-web and swirl.

With this feature, it's expected that these crates would have `pub
extern crate serde;` in some accessible path, and add
`#[serde(serde_path = "that_crate::wherever::serde")]` anywhere they
place serde's derives. Those crates would also have to derive
`that_crate::whatever::serde::Deserialize`, or `use` the macros
explicitly beforehand.

The test for this is a little funky, as it's testing this in a way that
is not the intended use case, or even one we want to support. It has its
own module which re-exports all of serde, but defines its own
`Serialize` and `Deserialize` traits. We then test that we generated
impls for those traits, instead of serde's. The only other way to test
this would be to create a new test crate which does not depend on serde,
but instead depends on `serde_derive` and a third crate which publicly
re-exports serde. This feels like way too much overhead for a single
test case, hence the funky test given.

I didn't see anywhere in this repo to document this attribute, so I
assume the docs will have to be done as a separate PR to a separate
repo.

Fixes #1487

serde_derive/src/de.rs

@@ -60,7 +60,7 @@ pub fn expand_derive_deserialize(input: &syn::DeriveInput) -> Result<TokenStream
         }
     };
 
-    Ok(dummy::wrap_in_const("DESERIALIZE", ident, impl_block))
+    Ok(dummy::wrap_in_const(cont.attrs.serde_path(), "DESERIALIZE", ident, impl_block))
 }
 
 fn precondition(cx: &Ctxt, cont: &Container) {