manifest: packages processing should match yunohost

[selfhoster1312]

Since #118 was merged yesterday, i believe there is a mismatch between how package_linter and yunohost process the resources.apt.packages list in a string form.

From yunohost:

self.packages = [value.strip() for value in self.packages.split(",")]

In practice it should not change much because apt will gladly eat a space-separated string as multiple package arguments, but i'm not sure if it could affect something else.