Fix realpath function

[remorses]

Fix #627

I use panic when there is an error in eval symlinks, this will panic when there is a cycle of symblic links, previously it would try to resolve the links forever˙

[evanw]

Thanks for the fix. I'm removing the panic because esbuild shouldn't crash in this case. I added a comment with more rationale about why the error is ignored.

Unfortunately I was unable to reproduce the infinite loop case, neither in the old code nor in the new code. I just get ELOOP. Regardless there is now test coverage for a symlink cycle so it should catch problems if they come up in the future.