lib.cleanSourceFilter: Fix VIM swap file filtering

The backslash wasn't properly escaped, and "\." is apparently equal to
".". So it's accidentally filtering out these valid file names (in
Nixpkgs):

trace: excluding clfswm
trace: excluding larswm
trace: excluding mkpasswd

While at it, turn the file filter stricter to what it was before
e2589b3c. That is, the file name must
start with a dot: '.swp', '.foo.swo' are filtered but 'bar.swf' is not.