swift-package-manager

Rename the `symlink()` function that doesn't have POSIX semantics to
`createSymlink()` to be a bit less confusing and also match the other
freestanding filesystem-related functions.  Also transition away from
the <b>other<b> `symlink()` function that also doesn't have POSIX
semantics.  That function's API never made sense anyway since it took
<i>three</i> paths, the last one being the root relative to which the
paths should be made relative.  But this doesn't make sense, because
a symlink with a relative target path must necessarily be relative to
the directory containing the symlink (that's how they work).

This also lets us get rid of one more function from the POSIX module.