Fix and refactor installation workflow

[siddhpant]

• Use correct library name in requirements.txt.
• Fix distro dependent conditional and dependencies.
• Remove stale things (like trying to run non-existent replace.sh).
• There are no .sh files in src top-level. Hence, use shopt in debian/postint for setting perms in sub-directories.
• Use better bash constructs.
• Better formatting for eyes including 80 char limit.

Since I am on Debian, the distro dependent conditional needs some testing, but it is better than the previous one (for eg. it used pamac instead of pacman for Arch).

[slimbook]

Thank you for contributing! Looks good ;)