Fix loadbefore, soft, and normal dependencies with spaces. Fixes BUKKIT-5418

This change makes the lists of loadbefore, softdependency, and dependency
replace the spaces in the names with underscored to reflect the behavior
used with names.
Provide warnings for spaces in plugin names. Addresses BUKKIT-5419
Update data folder migration for spaces in plugin names. Fixes BUKKIT-5417

This change drops the previous plugin data folder migration based on the
plugin's file name, and adapts the migration to now instead consider
plugins that have spaces in their original name.