Significantly simplify get_core_checksums(), as the caching and chunking was causing too much grief.

Make sure we only do our pre-flight is_writable check when the file exists.

see #18201. see #22704.


git-svn-id: https://develop.svn.wordpress.org/trunk@25801 602fd350-edb4-49c9-b593-d223f7449a82