docs(ref): Rename DEP_NAME_KEY to DEP_LINKS_KEY

`NAME` is unclear what should go here and the docs have to specifically
call out that it is `package.links` and not `package.name`.
The hope with this rename is to make this even clearer,
especially if we move forward with #3544 and add an alternative to
`DEP_NAME_KEY` that uses the dependency name.

Author: epage

src/doc/src/reference/build-scripts.md

@@ -475,7 +475,7 @@ convention of native-library-related packages:
 
 * Common dependencies on `foo-sys` alleviates the rule about one package per
   value of `links`.
-* Other `-sys` packages can take advantage of the `DEP_NAME_KEY=value`
+* Other `-sys` packages can take advantage of the `DEP_LINKS_KEY=value`
   environment variables to better integrate with other packages. See the
   ["Using another `sys` crate"][using-another-sys] example.
 * A common dependency allows centralizing logic on discovering `libfoo` itself

src/doc/src/reference/environment-variables.md

@@ -383,7 +383,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
   [`release`] profile. Using this environment variable is not recommended.
   Using other environment variables like `OPT_LEVEL` provide a more correct
   view of the actual settings being used.
-* `DEP_<name>_<key>` --- For more information about this set of environment variables,
+* `DEP_<links>_<key>` --- For more information about this set of environment variables,
   see build script documentation about [`links`][links].
 * `RUSTC`, `RUSTDOC` --- the compiler and documentation generator that Cargo has
   resolved to use, passed to the build script so it might use it as well.