Rename `source` to `span` and `span` to `source`
- Rename `clean::Item.source` to `span`
- Rename `clean::Span::span()` to `clean::Span::inner()`
- Rename `rustdoc_json_types::Item.source` to `span`
- rustdoc-json: Rename `Import.span` to `Import.source`
*See also the [discussion on Zulip][z] (this is a bit more than discussed in
that conversation, but all the changes are related).*
r? `@jyn514`
[z]: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/get.20span.20of.20file.20from.20name/near/229603729
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