build: log build sources with -v (#9672)
With the changes I've been making to mypy's import handling, I think
this would be a useful thing to add preemptively.
Note that I would have found this very useful at points, and I think
others would too, eg #7672 and #8584
The existing logging ignores source_modules and source_text and won't
help with determining what mypy things the module name for a given file
is, which is useful for namespace package issues as in the complaint in #8584.
Co-authored-by: hauntsaninja <>
