Default cargo publish to the alt registry if it's the only allowed one

[canova]

Hi, this PR fixes #8036. Previously if we had only one allowed registry in Cargo.toml file, cargo publish was failing. But with this PR, we are defaulting to the only allowed registry with printing a note if there is only one registry in that array. I believe this will make things easier for people who use an alternative registry all the time.

[rust-highfive]

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[ehuss]

Thanks for the PR! Would you mind updating the documentation to take note of the behavior? I would add a small note to the bottom of the section https://github.com/rust-lang/cargo/blob/master/src/doc/src/reference/manifest.md#the-publish-field. And also update the --registry flag documentation for the cargo publish man page. The man pages have recently been rearranged, so you'll need to rebase your PR to the lastest master. The source is in https://github.com/rust-lang/cargo/blob/master/src/doc/man/cargo-publish.md. I would remove {{> options-registry }} which loads a generic template for the --registry flag and add a custom definition, that might look something like this:

{{#option "`--registry` _registry_"}}
Name of the registry to publish to. Registry names are defined in [Cargo
config files](../reference/config.html). If not specified, and there is a
[`package.publish`](../reference/manifest.html#the-publish-field) field in
`Cargo.toml` with a single registry, then it will publish to that registry.
Otherwise it will use the default registry, which is defined by the
[`registry.default`](../reference/config.html#registry-default) config key
which defaults to `crates-io`.
{{/option}}

You can then run the build-man.sh script to rebuild the man pages.

[canova]

Thanks @ehuss, updated the code! Rebased on top of current master and added two more commits for the doc/man page update you mentioned. Could you take a look?

[ehuss]

Thanks!

@bors r+

[bors]

📌 Commit a8ce734 has been approved by ehuss

[bors]

⌛ Testing commit a8ce734 with merge 449743b...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing 449743b to master...