How can I `source` a file on my path and have it tab-complete?
You can put this into your bashrc:
complete -c source-c stands for command names which will effectively search in PATH. But that requires that the files are executable. You could elaborate on that and program a custom completion function which uses find to retrieve all shell scripts in PATH.
You can use bash built-in commands "complete, compgen, compopt" to do a customized completion for "source".
Link --> https://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html