fix cabal install --program-suffix/prefix (fix #10290 and #10476)

When checking for existing installations, cabal would not account for
an affix (suffix or prefix). So, if you had a `hello` binary installed, installing a
second one with a non-empty affix (a perfectly legal operation) would
fail.

The reason seemed to be a typo in
09c04e9, which passed the arguments to
the Symlink structure in a wrong order.

When failing to install a binary because of an existing one, cabal would
report suffix-less existing target even if a suffix was set.

cabal-install/src/Distribution/Client/CmdInstall.hs

@@ -1126,8 +1126,8 @@ symlink
       overwritePolicy
       installDir
       (mkSourceBinDir unit)
-      (mkExeName exe)
       (mkFinalExeName exe)
+      (mkExeName exe)
 
 -- |
 -- -- * When 'InstallCheckOnly', warn if install would fail overwrite policy
@@ -1172,7 +1172,7 @@ installCheckUnitExes
       errorMessage installdir exe = case overwritePolicy of
         NeverOverwrite ->
           "Path '"
-            <> (installdir </> prettyShow exe)
+            <> (installdir </> mkFinalExeName exe)
             <> "' already exists. "
             <> "Use --overwrite-policy=always to overwrite."
         -- This shouldn't even be possible, but we keep it in case symlinking or