remove `LinkMeta` from `SharedCrateContext`

remove `LinkMeta` from `SharedCrateContext`

A number of things were using `crate_hash` that really ought to be using
`crate_disambiguator` (e.g., to create the plugin symbol names). They
have been updated.

It is important to remove `LinkMeta` from `SharedCrateContext` since it
contains a hash of the entire crate, and hence it will change
whenever **anything** changes (which would then require
rebuilding **everything**).