mlos_bench: shell command line parsing improvements

[bpkroth]

Adds support for splitting a single shell command line into subcommands and applying the first token script name full path resolution logic to it.
Also cleans up slightly the handling of . directories from #459.
For example, in the following:

. env.sh && partial/path/to/app/env/script.py || another/script.sh

. should be left alone as equivalent to the shell keyword source, and both scripts should resolved to an expanded full path, with the python script prefixed with a python executable path.

Tests for both are included.

[motus]

Looks great! We can add more improvements later