Do not emit every iface load/codegen event to stderr when using -v by default

Motivation

Not only does it clutter the output horribly, it also seems to overly much impact performance.

ghc 8.6.5 normal: 2.4s
ghc 8.6.4 -v    : 2.5s

ghc head normal : 2.4s
ghc head -v     : 4s

This seems a bit excessive, especially when we don't actually write an eventlog.

I suspect the main cost is all the IO (44k lines for head!) so just avoid this output by default while still writing to the eventlog is likely fine.

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