Problem:
When disabling, the loader list is traversed forwards while entries are
removed. As a result, some entries are skipped and remain active. Thus,
disabling `vim.loader` restores `_G.loadfile`, but does not fully
restore the original package loader chain.
Solution:
Traverse `package.loaders` in reverse when removing cached loaders.
Capture the original `package.loaders` list in the test, and assert that
the list is restored properly.
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