rustdoc: get back missing crate-name when --playground-url is used
follow up PR #37763
r? @alexcrichton (since you r+ed to #37763 )
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Edit: When `#![doc(html_playground_url="")]` is used, the current crate name is saved to `PLAYGROUND`, so rustdoc may generate `extern crate NAME;` into code snips automatically. But when `--playground-url` was introduced in PR #37763, I forgot saving crate name to `PLAYGROUND`. This PR fix that.
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Update:
- add test
- unstable `--playground-url`
827eba4e70