Imports that use hyphen in filename break when transpiled to Lua

example:

```ts
import { vectorlight } from "lib-defs/hump/vector-light";
```

transpiles to

```lua
local __TSTL_vector-light = require("lib-defs.hump.vector-light");
local vectorlight = __TSTL_vector-light.vectorlight;
```

which throws the error:

```
expected statement near '-'
```