Ensure that the file relative to the example scripts is called.
Previously, the unqualified call would give precedence to source files in
PATH. This could lead to the wrong file being called when dir
`helper` (containing another file named `start-bash-session.sh`) was in PATH.
- The scripts now work within arbitrary nix-shells.
Previously, they failed when run from nix shells other than
`examples/shell.nix`.
- The scripts now work from arbitrary working dirs.
Move this feature from a module preset to a regular option, so that it's
easily discoverable and accessible.
Simplify the implementation of `generateSecrets` by adding it to the
existing `setup-secrets` service script.
Also rename option setup-secrets -> setupSecrets.
Previously, the sessions contained only explicitly exported variables
and functions.
This was fragile and in part buggy due to lacking exports.
Interactive features like user-defined aliases and functions are still
working as before.
