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/internals/attr.rs

@@ -218,6 +218,7 @@ pub struct Container {
     remote: Option<syn::Path>,
     identifier: Identifier,
     has_flatten: bool,
+    serde_path: Option<syn::Path>,
 }
 
 /// Styles of representing an enum.
@@ -298,6 +299,7 @@ impl Container {
         let mut remote = Attr::none(cx, "remote");
         let mut field_identifier = BoolAttr::none(cx, "field_identifier");
         let mut variant_identifier = BoolAttr::none(cx, "variant_identifier");
+        let mut serde_path = Attr::none(cx, "serde_path");
 
         for meta_items in item.attrs.iter().filter_map(get_serde_meta_items) {
             for meta_item in meta_items {
@@ -582,6 +584,13 @@ impl Container {
                         variant_identifier.set_true(word);
                     }
 
+                    // Parse `#[serde(serde_path = "foo")]`
+                    Meta(NameValue(ref m)) if m.ident == "serde_path" => {
+                        if let Ok(path) = parse_lit_into_path(cx, &m.ident, &m.lit) {
+                            serde_path.set(&m.ident, path)
+                        }
+                    }
+
                     Meta(ref meta_item) => {
                         cx.error_spanned_by(
                             meta_item.name(),
@@ -613,6 +622,7 @@ impl Container {
             remote: remote.get(),
             identifier: decide_identifier(cx, item, field_identifier, variant_identifier),
             has_flatten: false,
+            serde_path: serde_path.get(),
         }
     }
 
@@ -671,6 +681,10 @@ impl Container {
     pub fn mark_has_flatten(&mut self) {
         self.has_flatten = true;
     }
+
+    pub fn serde_path(&self) -> Option<&syn::Path> {
+        self.serde_path.as_ref()
+    }
 }
 
 fn decide_tag(