go: Apply package exclusion equally

The exclusion logic was moved out of getGoDirs but only buildPhase was updated
causing checkPhase to possibly fail. This happened in golint as it has go
files in testdata that are meant as testdata files and not go packages to
test which caused the checkPhase to fail.

This is a port of ddcc7c67d2

builder/default.nix

@@ -298,7 +298,7 @@ let
             if [ -n "$subPackages" ]; then
               echo "$subPackages" | sed "s,\(^\| \),\1./,g"
             else
-              find . -type f -name \*$type.go -exec dirname {} \; | grep -v "/vendor/" | sort --unique
+              find . -type f -name \*$type.go -exec dirname {} \; | grep -v "/vendor/" | sort --unique | grep -v "$exclude"
             fi
           }
 
@@ -315,7 +315,6 @@ let
               export NIX_BUILD_CORES=1
           fi
           for pkg in $(getGoDirs ""); do
-            grep -q "$exclude" <<<$pkg && continue
             echo "Building subPackage $pkg"
             buildGoDir install "$pkg"
           done