Auto merge of #30043 - arielb1:split-metadata, r=nikomatsakis

This improves bootstrap times because of better parallelism - though I need to measure how much - and allows metadata to be modified without triggering a full recompile. This also ensures that metadata handling and the rest of rustc remain decoupled, which is a first step for switching to a new metadata format.

This is a [breaking-change] to all plugin authors because of the following renames:
 * `rustc::plugin` is now `rustc_plugin`
 * `rustc::metadata` is now `rustc_metadata`
 * Most data types from `rustc::metadata`, along with `LOCAL_CRATE`, are now in `rustc::middle::cstore`.
 * The CStore methods were split between the `rustc::middle::CrateStore` trait (and trait object) and the `rustc_metadata::cstore::CStore`, with an `Rc<CrateStore>` stored in the `Session`. The inner `CStore` can be accessed via the inner `Any` bound, but this is deprecated.

r? @nikomatsakis

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