Dependencies are always built with exact version contraints

Any dependencies are passed to jsii by reading the version out of the module's `package.json` file. This results in an exact version being required. This makes it more difficult to use the generated code with other libraries which may use a different version of the dependency.

Changing `Options.dep` to `{ [module: string]: string }` would allow specifying a SemVar version for each module dependency. Could instead add a new option or union type to maintain compatibility.