Normalize cwd with realpath to avoid casing mismatch

[swjcpy]

Summary

• Normalize the working directory using realpath to ensure correct filesystem casing
• Prevent false case-sensitivity errors on case-insensitive filesystems

Issue

Fixes #220

[sindresorhus]

Can you explain how that fixes the issue exactly? And it needs a test.

[swjcpy]

Thanks for the review! This change normalizes the cwd with realpathSync() in the XO constructor, so a symlinked or case-variant path resolves to the filesystem’s canonical path. That keeps ESLint’s internal cwd aligned with how it reports file paths, which avoids import/no-unresolved false positives on case-insensitive filesystems when the user’s path casing doesn’t match the actual path casing. I’ve also added a test that creates a symlink to the fixture project, instantiates XO with the symlink path, and asserts that linterOptions.cwd is the realpath.