Support for variable and constant exports for packages in the compiler

[roman-khimov]

Given the following foo:

package foo

const Const = 42

// NewBar return an integer \o/
func NewBar() int {
        return 10
}

This test should work:

func TestImportConst(t *testing.T) {
       src := `
               package importconst

               import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/foo"

               func Main() int {
                       return foo.Const
               }
       `
       eval(t, src, big.NewInt(42))
}

But it doesn't and it also fails if we're to change foo to export a variable. It's a bit complicated to fix because traverseGlobals is assumed to be called once as it emits an INITSSLOT opcode and convertGlobals shouldn't ignore constants for subpackages. ast.SelectorExpr handling at line 808 of codegen.go also should be made aware of package-level constants and variables.

It's also somewhat related to #1150 as at the moment these variables/constants would end up in the same namespace.

[fyrchik]

Somewhat depends on #1228 , where Main func is no longer special.