Update alias to use legacyAlias

[rondale-sc]

This package relies on alias behavior from pre 1.6 Ember..

emberjs/ember.js#9265 (comment)

This replicates the old behavior by implementing alias as a computed. In the main application where ember-charts is used this is already being done, and this package on the whole may not even work without this change.

In order to get the codemod I wrote to fix the alias I needed to first run https://github.com/ember-codemods/ember-modules-codemod which converted the global use to the module import use

[rondale-sc]

@mixonic We're not the only consumers of this (presumably) which may mean that the legacyAlias will behave differently for those consumers. However, I'm fairly certain that this addon doesn't work at all without this change (to legacyAlias).