Switch Effective_license_text from []string to Paths

Effective_license_text contains paths to files that are copied from
one module to another and so need to be converted to Paths within the
context of the owning module as the paths are relative to the owning
module's directory.

The previous code did convert the license_text property to paths but
converted it back to strings again which was confusing and does not
follow the normal pattern.

Bug: 181569894
Test: m nothing
Change-Id: Iea09ee7f3de1187a2c3e41455ca83b0233d904b2