Correctly handle installationMethod when using bundled build

[Daniel15]

Summary
We do require('package.json') in a few places to get the version number and installation method of Yarn. In the Webpack build, this is embedded within the JS file itself. While this works fine for the version number (as that's constant), the installation methods can be different even with an identical build of Yarn. For example, Homebrew takes our tarball and then patches the installationMethod in package.json so we can tell that Yarn was installed via Homebrew. Using require('package.json') in this case won't reflect the update as it's only seeing package.json at the time of build (rather than at runtime).

Instead, we need to actually read the package.json file to check for the installationMethod at runtime. This looks in the parent directory (../package.json) due to the directory structure:

\package.json  <--- This is the file we want
\lib
  \yarn-cli.js <--- This is where the code is
\bin
  \yarn
  \yarn.cmd

Test plan
.\bin\yarn --version, ensure it says 0.24.0-0
.\bin\yarn install. ensure heading says "Yarn install v0.24.0-0"
.\bin\yarn versions, ensure
.\scripts\build-webpack.js then do all the same tests with .\artifacts\yarn-0.24.0.js

Closes #3085

[bestander]

why sync?
We use async/await in all the other places, don't we?

[Daniel15]

The async version of existsSync is deprecated.

cli/index.js also uses existsSync and readFileSync:

yarn/src/cli/index.js

Lines 296 to 297 in 45af656

	const possibleLoc = path.join(config.cwd, registries[registryName].filename);
	const manifest = fs.existsSync(possibleLoc) ? fs.readFileSync(possibleLoc, 'utf8') : 'No manifest';

Having said that, I can change reading the file to be async by creating a getInstallationMethod async function and using that where needed.

[bestander]

I think this line broke a test

[Daniel15]

Oops - I thought I ran the tests locally, but I might have only run the linter :/ I'll fix it.

[bestander]

Hey @Daniel15, this needs a rebase

[Daniel15]

Sure, I'll do that soon :)

[Daniel15]

Rebased, I'll merge once I verify that the tests pass.