compiler: move tests from vm/tests

These don't belong to VM as they compile some Go code and run it in a VM. One
may call them integration tests, but I prefer to attribute them to
compiler. Moving these tests into pkg/compiler also allows to properly count
the compiler coverage they add:

-ok     github.com/CityOfZion/neo-go/pkg/compiler       (cached)        coverage: 69.7% of statements
+ok     github.com/CityOfZion/neo-go/pkg/compiler       (cached)        coverage: 84.2% of statements

This change also fixes `contant` typo and removes fake packages exposed to the
public by moving foo/bar/foobar into the testdata directory.

pkg/compiler/import_test.go

@@ -0,0 +1,50 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestImportFunction(t *testing.T) {
+	src := `
+		package somethingelse
+
+		import "github.com/CityOfZion/neo-go/pkg/compiler/testdata/foo"
+
+		func Main() int {
+			i := foo.NewBar()
+			return i
+		}
+	`
+	eval(t, src, big.NewInt(10))
+}
+
+func TestImportStruct(t *testing.T) {
+	src := `
+	 	package somethingwedontcareabout
+
+		import "github.com/CityOfZion/neo-go/pkg/compiler/testdata/bar"
+
+	 	func Main() int {
+			 b := bar.Bar{
+				 X: 4,
+			 }
+			 return b.Y
+	 	}
+	 `
+	eval(t, src, []byte{})
+}
+
+func TestMultipleDirFileImport(t *testing.T) {
+	src := `
+		package hello
+
+		import "github.com/CityOfZion/neo-go/pkg/compiler/testdata/foobar"
+
+		func Main() bool {
+			ok := foobar.OtherBool()
+			return ok
+		}
+	`
+	eval(t, src, big.NewInt(1))
+}