GHCi: Fix order of `PackageDBFlag`s for interactive home unit

Hannes Siebenhandlrequested to merge
wip/fendor/ghc-ghci-mhu-27640 into master

Fixes #27640 (closed)

Reverse the package db stacks before calculating the longest common prefix.

Adds a regression test so this doesn't happen again. Verified this test fails without the patch.

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