Support evaluating just an expression

Occasionally it can be useful to evaluate just an expression.
Currently, `pkl eval -x` requires to also specify a module.
A cross-platform workaround is `pkl eval pkl:base -x`.
However, it would be nice if `pkl eval -x` just worked.

Start at the command handling for `pkl eval -x`, which currently requires a module and uses `pkl:base` as a workaround. Check how the CLI parses the expression and module arguments, then verify that an expression-only invocation works without breaking module-based evaluation.

Written by the indexing model from the issue text.