Fix for installing CscopeSublime from PackageControl on ST3

@vanrijn This is a fix for https://github.com/ameyp/CscopeSublime/issues/36.

Package Control on ST3 compresses the package into a single "package-name.sublime-package" file, but ST3 internally treats the location of the package's contents as being in "Packages/packages-name/". Hence, strip out the ".sublime-package" (if it exists) from the file-path.