refactor: src instead of url

[cachho]

This PR is renaming url variables to src in appropriate placse.

There are two reasons for this PR.

1. We use local data too. In add and add_local this is reflected by naming the arguments url and content. So naming all data sources url in the following functions is confusing to read when you debug local data.
2. The BaseChunker has url as a reused variable. It did not cause problems, but should still be avoided.