Confusing Error Message if package-name is omitted

I compiled a library as a GHC package without adding the appropriate package-name flag to the ghc command line.
When trying to compile a module that used the package, ghc complained that it couldn't find package "Main".

This happens with the HEAD. With GHC 5.04 it (seemingly) was no problem to leave out the -package-name specifier.

Why is the -package-name flag needed at all?

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Trac field	Value
Version	None
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	ResolvedFixed
Component	Driver
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Confusing Error Message if package-name is omitted