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
e2589b3. That is, the file name must
start with a dot: '.swp', '.foo.swo' are filtered but 'bar.swf' is not.

lib/sources.nix

@@ -15,9 +15,10 @@ rec {
   cleanSourceFilter = name: type: let baseName = baseNameOf (toString name); in ! (
     # Filter out Subversion and CVS directories.
     (type == "directory" && (baseName == ".git" || baseName == ".svn" || baseName == "CVS" || baseName == ".hg")) ||
-    # Filter out backup files.
+    # Filter out editor backup / swap files.
     lib.hasSuffix "~" baseName ||
-    builtins.match "^.*\.sw[a-z]$" baseName != null ||
+    builtins.match "^\\.sw[a-z]$" baseName != null ||
+    builtins.match "^\\..*\\.sw[a-z]$" baseName != null ||
 
     # Filter out generates files.
     lib.hasSuffix ".o" baseName ||