* Use implicit Prelude
The previous behavior was introduced as a fix for #4464. It seems that
this change alone did not fix the issue, and `stack ghci` and `cabal
repl` only work with GHC 8.4.1 or newer, as no custom Prelude is loaded
for these versions. Given this, it seems cleaner to revert to the
implicit Prelude.
* PandocMonad: remove outdated check for base version
Only base versions 4.9 and later are supported, the check for
`MIN_VERSION_base(4,8,0)` is therefore unnecessary.
* Always use custom prelude
Previously, the custom prelude was used only with older GHC versions, as
a workaround for problems with ghci. The ghci problems are resolved by
replacing package `base` with `base-noprelude`, allowing for consistent
use of the custom prelude across all GHC versions.
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