Convered all Object based key-value stores to Maps

[mfrederiksen]

I have updated all key-value Objects (specifically, packages and exportNames in dependency-report.js and filepathsForExports in package.js) to use Map.
This is to resolve problems when names or packages conflict with keys from Object.prototype.

Examples of this problem are lines like:
import toString from ‘lodash/toString’

This would result in the key-value stores not getting initialized because:
if (!this.filepathsForExports['toString']) {
will never evaluate false.

Methods on DependencyReport and Package were updated to ensure consistency with the existing API