Emit info-level log message when package envs are loaded

A common complaint with the new package environment files feature is
that it's not obvious when package environments have been picked up.
This patch applies the same strategy that was already used for `.ghci` files
(which exhibit similar potential for confusion, c.f. #11389) to package
environment files.

For instance, this new notification looks like below for a GHCi invocation which
loads both, a GHCi configuration as well as a package environment:

  GHCi, version 8.5.20180512: http://www.haskell.org/ghc/  :? for help
  Loaded package environment from /tmp/parsec-3.1.13.0/.ghc.environment.x86_64-linux-8.5.20180512
  Loaded GHCi configuration from /home/hvr/.ghci
  Prelude>

Addresses #15145

Reviewed By: bgamari, angerman

GHC Trac Issues: #15145

Differential Revision: https://phabricator.haskell.org/D4689

compiler/main/DynFlags.hs

@@ -4959,6 +4959,9 @@ interpretPackageEnv dflags = do
         return dflags
       Just envfile -> do
         content <- readFile envfile
+        putLogMsg dflags NoReason SevInfo noSrcSpan
+             (defaultUserStyle dflags)
+             (text ("Loaded package environment from " ++ envfile))
         let setFlags :: DynP ()
             setFlags = do
               setGeneralFlag Opt_HideAllPackages