Renam namespace--find-name to namespace--lookup.

* namespace.el (namespace--lookup): Renamed and update callers.
* namespace-tools.el:
Some tests for name conflicts.

* namespace-test.el (test-conflict.1, test-conflict.2)
(test-conflict.3):  New tests.

* namespace.el (namespace-error, namespace-conflict)
(namespace-use-conflict, namespace-import-conflict)
(namespace-export-conflict): New errors.
(namespace--error, namespace--conflicts-to-csyms): new helper
(namespace--conflict): New.
(namespace--check-use-conflicts)
(namespace--check-import-conflicts)
(namespace--check-export-conflicts): Use it
(namespace--check-disjoint): It's :test not :key.
(namespace--find-qsym-or-lose): Fix type decl.
(namespace--name-external-p, namespace--defun-like): Add catch-all
clauses to pcase to guard against unintended fall-through.