feat(private git templates): addresses #8166 and builds on #8167

Created by: jasonleehodges

• Attempting to address #8166 and building on the feedback provided in #8167.
• Added a test to show that the getTemplateInstallPackage actually returns what is expected.
• Getting some errors when actually trying to install a private package from github, however, and could use some guidance on how best to troubleshoot:

Require stack:
- /Users/jasonhodges/Documents/apps-tools/testCRA/test-cra2/node_modules/react-scripts/scripts/init.js
- /Users/jasonhodges/Documents/apps-tools/testCRA/test-cra2/[eval]
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:965:15)
    at Function.resolve (internal/modules/cjs/helpers.js:78:19)
    at module.exports (/Users/jasonhodges/Documents/apps-tools/testCRA/test-cra2/node_modules/react-scripts/scripts/init.js:110:13)
    at [eval]:3:14
    at Script.runInThisContext (vm.js:120:18)
    at Object.runInThisContext (vm.js:309:38)
    at Object.<anonymous> ([eval]-wrapper:10:26)
    at Module._compile (internal/modules/cjs/loader.js:1137:30)
    at evalScript (internal/process/execution.js:94:25)
    at internal/main/eval_string.js:23:3 {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/Users/jasonhodges/Documents/apps-tools/testCRA/test-cra2/node_modules/react-scripts/scripts/init.js',
    '/Users/jasonhodges/Documents/apps-tools/testCRA/test-cra2/[eval]'
  ]
}

It's worth noting that I also tried pointing create-react-app at a tarball template as one of the unit test suggests, which could be an alternative solution to #8166, but that yielded an error of "A template was not provided." So I'm not sure if this is all behaving as intended.