Fix writing an absolute path to modulePaths in Jest config when ejecting

[mkarajohn]

Resolves #8965

Problem & Cause

As described in the linked issue, when a CRA project is ejected, the modulePaths entry of the jest config includes an array of absolute paths, if a baseUrl is defined in the tsconfig.json or jsconfig.json.

The array is created here exported as additionalModulePaths.

Since this exported array of additionalModulePaths is also used in the webpack.config.js here, where absolute paths are in fact needed, we cannot be directly modifying the way the additionalModulePaths array contents are created.

Solution

What this PR does in order to fix the issue, is to translate the absolute paths to relative ones in the createJestConfig.js file here, since adding absolute paths in the modulePaths is problematic, in cases such as CI pipelines, or simply someone else trying to run the tests on their own machine. Making them relative to the rootDir solves any issues.

Before:

After:

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[mkarajohn]

I realized that the original fix would not work for paths longer than 1 directory deep, e.g. foo/bar as it was simply using path.basename() which, in such cases, does not translate to the correct path.

So, createJestConfig.js had to have some way to know the original baseUrl path, in order to translate it relative to rootDir. As such, a little change was made in ec12ba1, to the output of getAdditionalModulePaths(). It now returns an object like

type additionalModulePaths = {
  resolved: [string];
  relative: [string];
}

additionalModulePaths.resolved is used in the webpack.config.js modules resolution, while additionalModulePaths.relative is used in createJestConfig.js. These are the only 2 places where additionalModulePaths is used at all.