compiler: initialize packages according to go spec

`init()` functions should be called during package initialization,
after global variables were processed.

pkg/compiler/analysis.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/nspcc-dev/neo-go/pkg/vm/emit"
 	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
+	"golang.org/x/tools/go/loader"
 )
 
 var (
@@ -47,8 +48,20 @@ func (c *codegen) traverseGlobals() int {
 			return 0
 		}
 		emit.Instruction(c.prog.BinWriter, opcode.INITSSLOT, []byte{byte(n)})
-		c.ForEachFile(c.convertGlobals)
-		c.ForEachFile(c.convertInitFuncs)
+		c.ForEachPackage(func(pkg *loader.PackageInfo) {
+			for _, f := range pkg.Files {
+				c.fillImportMap(f, pkg.Pkg)
+				c.convertGlobals(f, pkg.Pkg)
+			}
+			for _, f := range pkg.Files {
+				c.fillImportMap(f, pkg.Pkg)
+				c.convertInitFuncs(f, pkg.Pkg)
+			}
+			// because we reuse `convertFuncDecl` for init funcs,
+			// we need to cleare scope, so that global variables
+			// encountered after will be recognized as globals.
+			c.scope = nil
+		})
 	}
 	return n
 }

pkg/compiler/compiler.go

@@ -47,17 +47,25 @@ type buildInfo struct {
 	program        *loader.Program
 }
 
-// ForEachFile executes fn on each file used in current program.
-func (c *codegen) ForEachFile(fn func(*ast.File, *types.Package)) {
+// ForEachPackage executes fn on each package used in the current program
+// in the order they should be initialized.
+func (c *codegen) ForEachPackage(fn func(*loader.PackageInfo)) {
 	for i := range c.packages {
 		pkg := c.buildInfo.program.Package(c.packages[i])
 		c.typeInfo = &pkg.Info
 		c.currPkg = pkg.Pkg
+		fn(pkg)
+	}
+}
+
+// ForEachFile executes fn on each file used in current program.
+func (c *codegen) ForEachFile(fn func(*ast.File, *types.Package)) {
+	c.ForEachPackage(func(pkg *loader.PackageInfo) {
 		for _, f := range pkg.Files {
 			c.fillImportMap(f, pkg.Pkg)
 			fn(f, pkg.Pkg)
 		}
-	}
+	})
 }
 
 // fillImportMap fills import map for f.

pkg/compiler/init_test.go

@@ -80,4 +80,11 @@ func TestImportOrder(t *testing.T) {
 		func Main() int { return pkg3.A }`
 		eval(t, src, big.NewInt(1))
 	})
+	t.Run("InitializeOnce", func(t *testing.T) {
+		src := `package foo
+		import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/pkg3"
+		var A = pkg3.A
+		func Main() int { return A }`
+		eval(t, src, big.NewInt(3))
+	})
 }

pkg/compiler/testdata/pkg3/pkg3.go

@@ -1,3 +1,7 @@
 package pkg3
 
 var A int
+
+func init() {
+	A = 3
+}