refactor exec.ExecScript

[Jon]

exec.ExecScript is too tightly coupled with executing the platform-specific runner script. Genericize it to allow uru to run additional scripts that live in the .uru dir.

This will likely be needed to expand support for additional GEM_HOME scenarios.

• update exec.CreateScript to return the runner script's basename and rename to exec.CreateSwitcherScript

[Jon]

Refactor exec.CreateScript. Closes #49

Rename function for clarity and remove usage of its sibling func exec.ExecScript as its usage is unneeded.

→ <<cset e38025d64d34>>