Only lookup ghcversion.h file in the RTS include-dirs by default.

Sylvain Henryrequested to merge
hsyl20/ghc:hsyl20/ghcversion-25106 into master

The code was introduced in 3549c952. It used getPackageIncludePath which name doesn't convey that it looks into all include paths of the preload units too. So this behavior is probably unintentional and it should be ok to change it.

Fix #25106 (closed)

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