Minimized code structure and fixed minor issue

[ghost]

Changes

• Minimized some unnecessary lines of code.
• Solved prefixed file output path problem with os.path.join (basically, this will handle missing trailing slash of the specified prefix):

- output_file = installation_prefix + package_files[input_file_path]
+ output_file_path = os.path.join(installation_prefix, output_file_path)

Example case is, if the file entries in manifest are the following:

"files": {
  "script": ".local/bin/script",
  "LICENSE": ".local/share/LICENSES/package.txt"
}

And given the prefix is $HOME, then the former and latter will result into:

- /data/data/com.termux/files/home.local/bin/script
- /data/data/com.termux/files/home.local/share/LICENSES/package.txt
+ /data/data/com.termux/files/home/.local/bin/script
+ /data/data/com.termux/files/home/.local/share/LICENSES/package.txt