feat(frontends/lean/parser): improve Import command

- The extension does not have to be provided.
- It can also import Lua files.
- Hierachical names can be used instead of strings.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(examples/lean): use new Import command in examples

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): disable verbose messages when importing files

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>