Update installer to use locals, and fs functions

[SkyTheCodeMaster]

This PR:

• Localizes every variable
• Removes the use of io.open to check if a file exists, and instead uses fs.exists
• Changes instances of if expr==false then to if not expr then

[PentagonLP]

Hi! First of all, thank you for your contribution.
I can't just merge your pull request, because all the file functions and all the http functions do now have their own library (https://github.com/PentagonLP/fileutils and https://github.com/PentagonLP/httputils), but their code is still copy-pasted in this file because "pastebin run" does only allow one file to be downloaded. Your changes will be made there and then be manually merged back into the installer.
Also, the installer has been modified for the new upcoming ccpt version (https://github.com/PentagonLP/ccpt/blob/DevVersion-1.1/ccptinstall.lua), but it inherited many of the issues of the old one. Your changes will be made in the over there.
I'm a bit short of time rn, but this pull request will be left open until your changes are implemented, so that I don't forget :)
Again, thank you for contributing!
~PentagonLP

[PentagonLP]

Code is now implemented, will do some tesing tomorrow before closing the pull request.