Git submodules and package.txt outdated

[astirpe]

Module membership_variable_period is duplicated: is present in both vertical-association and addons-pre-oca git-submodules.
File package.txt refers to addons-pre-oca/membership_variable_period but it should be vertical-association/membership_variable_period instead.

Module mass_mailing_partner is in addons-pre-oca git-submodule but it's available in OCA repo: https://github.com/OCA/social/tree/15.0
The OCA repo social should be added as git-submodule and file package.txt should refer to social/mass_mailing_partner instead.