Auto merge of #3967 - nipunn1313:rerender_readme, r=Turbo87

admin/render_readmes.rs to correctly identify the readme path

Previously, it would try to get a field called `readme_file`
which never existed in the Cargo.toml manifest format. This
fixes it.

https://doc.rust-lang.org/cargo/reference/manifest.html#the-readme-field

I believe the confusion arose because the `cargo package` command
actually converts the readme contents -> readme field and the
Cargo.toml readme field -> `readme_file` field when making the
request to crates.io.

However, in the admin render_readme backfill, it is opening the
original Cargo.toml which has a `readme` field for the path.

I manually tested with a local backend/frontend and confirmed that
readme paths were being re-rendered correctly.

Manual test screenshots:

Before this change - after doing a rerender
![image](https://user-images.githubusercontent.com/1300387/135545532-5df746af-77a8-4140-99a8-74b09fe512a1.png)

After this change - after doing a rerender
![image](https://user-images.githubusercontent.com/1300387/135545434-9f82ee13-e909-4c1d-8544-8ed6023b7944.png)

Repro steps
Set up a local dev environment via the [docs/CONTRIBUTING.md](https://github.com/rust-lang/crates.io/blob/master/docs/CONTRIBUTING.md) instructions.
Publish a package (I used one called `innercrate`) (using above instructions) with nonroot README
```
➜  innercrate git:(master) ✗ cat docs/README.md
Check out [other file](./Otherfile.md)
➜  innercrate git:(master) ✗ cat docs/Otherfile.md
hello world
➜  innercrate git:(master) ✗ grep readme Cargo.toml
readme = "docs/README.md"
```

Re-render readmes
`cargo run --bin crates-admin -- render-readmes --crate innercrate`
Look at http://localhost:4200/crates/innercrate

Author: bors

src/admin/render_readmes.rs

@@ -213,31 +213,24 @@ fn get_readme(
     };
 
     let rendered = {
-        let path = format!(
-            "{}-{}/{}",
-            krate_name, version.num, manifest.package.readme?
-        );
+        let readme_path = manifest.package.readme.as_ref()?;
+        let path = format!("{}-{}/{}", krate_name, version.num, readme_path);
         let contents = find_file_by_path(&mut entries, Path::new(&path), version, krate_name);
         text_to_html(
             &contents,
-            manifest
-                .package
-                .readme_file
-                .as_ref()
-                .map_or("README.md", |e| &**e),
+            readme_path,
             manifest.package.repository.as_deref(),
         )
     };
     return Some(rendered);
 
-    #[derive(Deserialize)]
+    #[derive(Debug, Deserialize)]
     struct Package {
         readme: Option<String>,
-        readme_file: Option<String>,
         repository: Option<String>,
     }
 
-    #[derive(Deserialize)]
+    #[derive(Debug, Deserialize)]
     struct Manifest {
         package: Package,
     }