Rollup merge of #87035 - GuillaumeGomez:fix-implementors-display, r=notriddle

Rollup merge of #87035 - GuillaumeGomez:fix-implementors-display, r=notriddle

Fix implementors display

Part of https://github.com/rust-lang/rust/issues/86632.

This PR does a few things:
 * It fixes of the JS rendered implementors.
 * It generates anchors for JS rendered implementors to make it coherent with the others.
 * It adds a test to ensure that we won't have the same issue again.
 * It changes the way we render the rustdoc-gui crates to simplify it a bit and also to allow to have dependencies without going through compiletest.

Before:

![Screenshot from 2021-07-10 13-30-13](https://user-images.githubusercontent.com/3050060/125174172-b4048700-e1c3-11eb-8f0e-c46081371d4f.png)

After:

![Screenshot from 2021-07-10 21-11-15](https://user-images.githubusercontent.com/3050060/125174173-b49d1d80-e1c3-11eb-8740-1dbbff70c2eb.png)

I plan to add the `[src]` links in another PR because this one is already big enough.

cc `@Mark-Simulacrum` (for the bootstrap changes)

r? `@Nemo157`