Fix detection of directories on Windows

Function tools.delete performs a test to determine if a path is a file or a directory to select the appropriate delete command rmdir vs del.
The current test however results in rmdir being used on files too, which then results in a build abortion with error "The directory name is invalid.". 

E.g.
    if exist "c:\luarocks\share\lua\5.2\luasocket_3_0rc1_2-socket.lua\" ( echo "I am a folder" ) else ( echo "I am a file" )
wrongly prints "I am a folder" (tested on Windows 10)

Whereas 
    if exist "c:\luarocks\share\lua\5.2\luasocket_3_0rc1_2-socket.lua\*" ( echo "I am a folder" ) else ( echo "I am a file" )
correctly prints "I am a file" (tested on Windows 10)

This fixes #670

(Also see https://stackoverflow.com/a/1466528/1793220 )

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