This PR extracts the `ExtensionIndexedDocsProvider` implementation to
the `indexed_docs` crate.
To achieve this, we introduce a new `Extension` trait that provides an
abstracted interface for calling an extension. This trait resides in the
`extension` crate, which has minimal dependencies and can be depended on
by other crates, like `indexed_docs`.
We're then able to implement the `ExtensionIndexedDocsProvider` without
having any knowledge of the Wasm-specific internals of the extension
system.
Release Notes:
- N/A
This PR begins the process of making the backing infrastructure for the
`/rustdoc` command more generic such that it can be applied to
additional documentation providers.
In this PR we:
- Rename the `rustdoc` crate to `indexed_docs` as a more general-purpose
name
- Start moving rustdoc-specific functionality into
`indexed_docs::providers::rustdoc`
- Add an `IndexedDocsRegistry` to hold multiple `IndexedDocsStore`s (one
per provider)
We haven't yet removed the rustdoc-specific bits in the `DocsIndexer`.
That will follow soon.
Release Notes:
- N/A
