Eliminate dependencies on `GHC.Internal.Text.Read` within `ghc-internal`

Wolfgang Jeltschrequested to merge
wip/jeltsch/text-read-uncovering into master

This contribution gets rid of any and all dependencies on GHC.Internal.Text.Read within ghc-internal, in the hope that this will enable us to move considerable parts of reading-related code and then further code out of ghc-internal.

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