Auto merge of #23512 - cpud36:fix_incorrect_relative_path_in_webidl_plugin, r=Manishearth
Auto merge of #23512 - cpud36:fix_incorrect_relative_path_in_webidl_plugin, r=Manishearth

Fix `#[webidl_must_inherit]` building when cwd is not servo crate root

Manifest dir should be more stable than current dir.

<!-- Please describe your changes on the following line: -->
I was trying to embed servo.

The directory layout of mine was the following:
```
/
*-servo
*-src
*-Cargo.toml
...
```

When I tried to build, cargo reported an error, in `#[webidl_must_inherit]` (Os error: 2).

After some research I have found that it failed to find `.webidl` files, because it used a path relative to `current_dir`.

I replaced current dir with manifest dir to make it more reliable.

---
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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (Not applicable)

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- [ ] There are tests for these changes OR
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I am unsure if these changes require tests. If they do, how should they be implemented?

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emilio/servo,saneyuki/servo,splav/servo,pyfisch/servo,notriddle/servo,paulrouget/servo,notriddle/servo,larsbergstrom/servo,splav/servo,paulrouget/servo,emilio/servo,emilio/servo,larsbergstrom/servo,DominoTree/servo,pyfisch/servo,emilio/servo,saneyuki/servo,emilio/servo,saneyuki/servo,KiChjang/servo,notriddle/servo,saneyuki/servo,notriddle/servo,splav/servo,notriddle/servo,nnethercote/servo,KiChjang/servo,DominoTree/servo,emilio/servo,nnethercote/servo,pyfisch/servo,splav/servo,notriddle/servo,emilio/servo,DominoTree/servo,nnethercote/servo,nnethercote/servo,notriddle/servo,notriddle/servo,KiChjang/servo,nnethercote/servo,paulrouget/servo,DominoTree/servo,DominoTree/servo,nnethercote/servo,pyfisch/servo,DominoTree/servo,KiChjang/servo,notriddle/servo,saneyuki/servo,splav/servo,emilio/servo,larsbergstrom/servo,paulrouget/servo,saneyuki/servo,emilio/servo,splav/servo,larsbergstrom/servo,DominoTree/servo,larsbergstrom/servo,nnethercote/servo,paulrouget/servo,nnethercote/servo,DominoTree/servo,pyfisch/servo,DominoTree/servo,pyfisch/servo,pyfisch/servo,pyfisch/servo,paulrouget/servo,nnethercote/servo,KiChjang/servo,splav/servo,larsbergstrom/servo,KiChjang/servo,larsbergstrom/servo,saneyuki/servo,saneyuki/servo,splav/servo,splav/servo,KiChjang/servo,saneyuki/servo,KiChjang/servo,paulrouget/servo,splav/servo,pyfisch/servo,KiChjang/servo,KiChjang/servo,larsbergstrom/servo,paulrouget/servo,saneyuki/servo,emilio/servo,pyfisch/servo,DominoTree/servo,larsbergstrom/servo,paulrouget/servo,larsbergstrom/servo,paulrouget/servo,nnethercote/servo,notriddle/servo