rustdoc: Improve doctest pass's name and module's name

As the docs at the top of the file say, it is an overloaded pass and
actually runs two lints.

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -2,7 +2,7 @@ use crate::clean;
 use crate::core::DocContext;
 use crate::fold::{self, DocFolder};
 use crate::html::markdown::{find_testable_code, ErrorCodes};
-use crate::passes::doc_test_lints::{should_have_doc_example, Tests};
+use crate::passes::check_doc_test_visibility::{should_have_doc_example, Tests};
 use crate::passes::Pass;
 use rustc_hir as hir;
 use rustc_lint::builtin::MISSING_DOCS;