Use non-root python logger

My code does something akin to the following.

```
gin.add_config_file_search_path(folder1)
gin.add_config_file_search_path(folder2)
gin.parse_config_files_and_bindings(...)
```

I am now using `gin` from within code which is itself now packages as a pip-installable python package. Ever since executing this change, I now see logging output like the following:

```
INFO:root:system_path_file_exists:base_surface_registry.gin
ERROR:root:Path not found: base_surface_registry.gin
INFO:root:system_path_file_exists:natural.gin
ERROR:root:Path not found: natural.gin
INFO:root:system_path_file_exists:performance/dev.gin
ERROR:root:Path not found: performance/dev.gin
INFO:root:system_path_file_exists:disable_assets/no_creatures.gin
ERROR:root:Path not found: disable_assets/no_creatures.gin
INFO:root:system_path_file_exists:performance/fast_terrain_assets.gin
ERROR:root:Path not found: performance/fast_terrain_assets.gin
```
I would like to mute all gin-related logging, but it appears gin is using the root logging.info() rather than `logger = logging.getLogger(__name__); logging.info(...)` as is recommended.

Example: https://github.com/google/gin-config/blob/ddd964cebaeaadcb9d1a9b01eaabdfd11dbe03d9/gin/resource_reader.py#L55

Switching to the recommended getLogger(__name__) would allow me to `logging.getLogger("gin").setLevel(logging.CRITICAL)` to ignore these messages.

I could likely PR this if theres any chance of it being accepted.