moc: fix type mismatch in moc_data

[joanbm]

moc.py was recently refactored by f648cb607f1c ("py3: add new helper: get_replacements_list"). This causes it to fail when MOC is started, because the code has two variables:

• moc_data, which is a str.
• data, which is a dict.

But the refactor accidentally handles moc_data as a dict.

This commit fixes the issue, and unifies data and moc_data, using the same pattern that is already being used in cmus.py.