Write .cargo/config.toml in x vendor

[bjorn3]

x vendor hides the cargo vendor telling you what to write to .cargo/config.toml. to use the vendored sources, so without this PR it isn't obvious how to use the vendored sources.

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[clubby789]

Why not just print out the config info captured? That avoids the trailing

To use vendored sources, add this to your .cargo/config.toml for this project:

and avoids automatically overwriting a potentially existing config.toml.

[bjorn3]

I found this will working on vendoring support for -Zbuild-std. That will require two separate .cargo/config.toml files to be written. One in the root and one in library/.

[clubby789]

Sounds good - the broken output is a bit unfortunate but not the end of the world. r=me when green

[Kobzol]

I don't think people will appreciate when running x vendor might overwrite their .cargo/config.toml file. Bootstrap should just print what to do, or this behavior should be opt-in.

[Kobzol]

Why remove this? GenerateCopyright can still make use of the final path, instead of having to know about VENDOR_DIR, which is an implementation detail of the vendor step.

[bjorn3]

VendorOutput only ever contained the data that should be written to .cargo/config.toml.

[Kobzol]

I don't like overwriting the config.toml file on disk (as per my other comment), but if we did that, we can at least return the path to the vendor directory from the step, to avoid other steps reaching for VENDOR_DIR.