`tap package create`/`verify` should automagically make the paths in the package definition file Linux friendly

• Complete the work on tap package create started in !563 (merged):
   - Normalize all paths in the package definition file to be included in the package to be created so that they contain only /.
   - Notify the user of the conversion with a Warning.
• Implement something similar for tap package verify as suggested by @StefanHolst in !563 (comment 398599640)
• Make tap package verify independent of the launch directory: the rationale behind this is that the package definition files are already retrieved from the installation directory where the command being run is located. So, making the command work only when launched from the installation directory is pretty annoying and does not add any flexibility, since the current working directory isn't used to specify what installation directory should be verified.