Use conditional requests

[twodayslate]

When communicating with the GitHub team they recommended using conditional requests when parsing packages. This would help with any API rate limiting issues.

Currently whenever a package is viewed that is an API request.

swift-package-registry/lib/package.js

Line 15 in 74a380c

req.octokitWithPersonalAccessToken.repos.get({ owner: req.params.owner, repo: req.params.repo }).then(async function (repository) {

Instead it should be a database lookup, if the conditional request does not say there is an update.

[twodayslate]

If there is an update should parse the new package. tightly coupled with #19