Replace fs-extra emptyDir with mkdir and rm of each entry

Replaces fs-extra's emptyDir/emptyDirSync with a recursive mkdir of the directory followed by an rm of each entry readdir returns. Emptying the directory rather than replacing it is what fs-extra does, and it keeps the directory's own inode and permissions, which removing and recreating it would not. mkdir runs first so a missing directory is created, as emptyDir does when readdir fails. A path that isn't safe to evaluate twice is hoisted into a const first; since both need a statement to sit in, a call in expression position is left alone, as are callback-form calls.