Don't complain about 'export import =' in strict-export-declare-modifiers

[jakebailey]

Motivated by:

import implementation = require("./implementation");
import polyfill = require("./polyfill");
import shim = require("./shim");

type Implementation = typeof implementation;
type Polyfill = typeof polyfill;
type Shim = typeof shim;

declare namespace index {
    export import Flags = implementation.Flags;

    const implementation: Implementation;
    const polyfill: Polyfill;
    const shim: Shim;
}

declare function index(re: ThisParameterType<Implementation>): implementation.Flags;

export = index;

This errors on the export even though it's required to get the right behavior and also does not make all of the other namespace members unexported.