Improve error message for package$Klass conflict with Klass

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If I follow the warning recommendation and move a class out of a package object, then recompile without cleaning, I get a generic, tautological error message:

pkgobjection.scala:9: error: NoLongerImplementedError is already defined as class NoLongerImplementedError

It would be nicer to specify: "...as class NoLongerImplementedError in package object pkgobjection."

I've read and understood #4344, and I've read and misunderstood #4695.

The spec says I should never (for now) let this happen, and maybe it doesn't in a tooled environment.

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Imported From: https://issues.scala-lang.org/browse/SI-5760?orig=1
Reporter: @som-snytt
Affected Versions: 2.10.0
Other Milestones: 2.10.0

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@som-snytt said:
Coincidentally, if I had waited awhile and used a compiler with
8068e1208466ab17af40c2670230ba5ac9704e0c,
I would never have known to repackage my little Error class and encounter this situation until one day my compile failed and someone said:
"-Xlint would have told you not to do that".
Medical research has shown that repeated head slaps can cause mini-concussions, dementia and early demise.

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@som-snytt said:
scala/scala#593

However, I was too late to push my hack to partest. Let that be a lesson to us all.

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@som-snytt said:
adriaanm merged commit f406550