Add the ability to overide the out-dir settings passed to rust-doc

[NovaliX-Dev]

Problem

Quick definitions :

• "Rustdoc command" means the command which doesn't involve cargo, rustdoc only.
• "Cargo command" means the command which involve cargo, cargo rustdoc.

If we want to get the output of rustdoc to a certain location (to process them, for example), we can use the out-dir option on the rustdoc command to specify that location.

If we use the cargo command, cargo will set that option for us. That's a very good behavior in general, but that also means we can't override it with our value if we wanted to.

If we try (using for example the command cargo rustdoc -- --out-dir=<dir>), we will get that error :

error: Option 'out-dir' given more than once

If one want to override out-dir, he will have to use the rustdoc command directly, which means he will have to do all the work cargo can do. For example, handling all the source code, all the dependencies, etc. That adds a lot of overhead, especially since the rustdoc command is not obvious to set up correctly (at least that's my opinion).

Proposed Solution

Cargo should detect if the out-dir option is already present, and only fill it when it's not there.

Notes

If that feature is accepted, I'm willing to implement it myself.

[epage]

If we did this, we should probably tie this into #6790.