Refactor of the environment detection functions.

[jpodivin]

• Platform determination simplification
• Import of 're' moved to the top
• Module doc string
• _findSwiplPathFromFindLib doc string simplified
• Superfluous 'else' statement removed from the '_findSwiplFromExec'
• Common '_findSwiplFromExec' code factored out into local function
• stdout line processing simplified
• Inaccurate description of the '_findSwiplMacOSHome' rectified
• 'get_swi_ver' function which wasn't called from anywhere was removed
• Basic linting such as trailing white space removal, space insertion around operators

Tested on Fedora34.