Fixes #4665. make "-r <dir>" work with new import-first loading

[giltayar]

Description of the Change

When falling back on CJS when loading test files, ensure that ERR_UNSUPPORTED_DIR_IMPORT is supported so that -r <cjs-package>/dir will work. See #4665 for a more detailed description.

Benefits

This is a bug fix to return support to what was before enabled.

Possible Drawbacks

None

Applicable issues

#4665

• Mocha follows semantic versioning: http://semver.org

• Is this a breaking change (major release)? No

• Is it an enhancement (minor release)? No

• Is it a bug fix, or does it not impact production code (patch release)? Yes

[coveralls]

Coverage remained the same at 94.369% when pulling a6fd8d7 on giltayar:dir-import-fix into f033ff1 on mochajs:master.

[juergba]

@giltayar could there be more cases require can handle, but import doesn't? Requiring json files?

[giltayar]

@juergba I can check, but this is only for test files and --require files loaded by mocha, and not for all requires/imports, so I don't think JSON files are relevant, as even if we don't support loading JSON test files, nobody would care.