Emit info-level log message when package environments have been picked up

A common complaint with the new package environment files feature is that it's not obvious when package environments have be picked up. Even though this feature has been in GHC since GHC 8.0, now with cabal 2.2 turning on ghc env file generation, this getting wider exposure.

A similiar UI issue existed with GHCi and its .ghci files and the solution there was to emit a message informing the user that an implicit configuration file was picked up:

GHCi, version 8.5.20180512: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/hvr/.ghci
Prelude>

The same solution would greatly reduce the confusion with package environment files which have an even trickier lookup logic than .ghci files.

Trac metadata

Trac field	Value
Version	8.0.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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