PY-13537 Use configurable list of ignored classes instead of checkbox

We've decided that possibility to suppress warnings about bad
method names for *all* standard classes is too general and may be
non-obvious for user. After all if inspections is too bothering it's
simpler to disable it at all. Much better way is too allow to
configure black-list of concrete classes for descendants of which
such warnings will be suppressed (as already done for some other
inspections in PyCharm).
Do not ignore import element completely if qualifier was resolved to namespace package

Consider the case when we are trying to invoke auto completion in
"import foo.bar.<caret>". Previously (mistakenly) type of
qualifier "foo.bar" was PyImportedModuleType(bar), which could not be
successfully resolved unless there is some other top-level "bar" module
under project roots. Now it's correct PyImportModuleType(foo.bar).
Add missing test data for Py3CompletionTest.testImportNamespacePackage