ignore cpanfile.snapshot
refactor split/join
remove _normalized_var_name
refactor: extract _define_scalar
add _define_constant
change _validate_var_name to allow constant or scalar
refactor: extract value
fix first pass at providing scalars and constants

The first pass at providing scalars and constants didn't account for the
"constants" changing value if the scalar version (or environment
variable) was written to.  Now both always lookup value from environment
and then config.  The config is still memoized.
remove import_depth
only load files that exist
update test for new constant/mutable variables
auto-generated
update test for _load being moved to fetch