Relative filepaths get extended in the output

[kit-ty-kate]

When calling dead_code_analyzer --all _build/default/src/**/.*objs/, the typical output contains:

<the fullpath to your current directory>/_build/default/src/mod.mli:32: <some dead function>

I think it would be more pleasant to read and share output that didn't have <the fullpath to your current directory>/ at the very least.
We could also improve further and drop _build/default/ as well but that's optional i think.