Fix ESLint config around files w/ CommonJS imports (#14380)

The `parserOptions.sourceType` config option in `.eslintrc.js` is used
to distinguish files that use ESM imports vs. files that do not.
The `import` plugin, specifically the `import/ambiguous` rule behaves
differently depending on this value. If a file is marked with
`sourceType` of `"module"`, then this rule will attempt to ensure that
the file does indeed have ESM imports and/or exports; otherwise it does
nothing.

In other words, files that use CJS imports are *not* "modules" according
to this setting, and therefore should not be marked with a `sourceType`
of `"module"`. This means we do not have to turn off the
`import/ambiguous` rule, as it will no longer trip up on these types of
files.

.eslintrc.js

@@ -44,14 +44,6 @@ module.exports = {
         path.resolve(__dirname, '.eslintrc.babel.js'),
         path.resolve(__dirname, '.eslintrc.typescript-compat.js'),
       ],
-      parserOptions: {
-        sourceType: 'module',
-      },
-      rules: {
-        // This rule does not work with CommonJS modules. We will just have to
-        // trust that all of the files specified above are indeed modules.
-        'import/unambiguous': 'off',
-      },
       settings: {
         'import/resolver': {
           // When determining the location of a `require()` call, use Node's