[utils.py] gathering/refactoring into a "utils/" folder (#6095)

* [utils] gathering/refactoring into a "utils/" folder

Moving current utils.py into utils/core.py and moving other *util*
modules under this new "utils/" as well.

Following steps include eroding at "utils/core.py" and breaking it down
into smaller modules.

* Improve tests

* Make loading examples in scope for tests

* Remove test class attrs examples_loaded and requires_examples

superset/utils/import_datasource.py

@@ -0,0 +1,76 @@
+# pylint: disable=C,R,W
+import logging
+
+from sqlalchemy.orm.session import make_transient
+
+
+def import_datasource(
+        session,
+        i_datasource,
+        lookup_database,
+        lookup_datasource,
+        import_time):
+    """Imports the datasource from the object to the database.
+
+     Metrics and columns and datasource will be overrided if exists.
+     This function can be used to import/export dashboards between multiple
+     superset instances. Audit metadata isn't copies over.
+    """
+    make_transient(i_datasource)
+    logging.info('Started import of the datasource: {}'.format(
+        i_datasource.to_json()))
+
+    i_datasource.id = None
+    i_datasource.database_id = lookup_database(i_datasource).id
+    i_datasource.alter_params(import_time=import_time)
+
+    # override the datasource
+    datasource = lookup_datasource(i_datasource)
+
+    if datasource:
+        datasource.override(i_datasource)
+        session.flush()
+    else:
+        datasource = i_datasource.copy()
+        session.add(datasource)
+        session.flush()
+
+    for m in i_datasource.metrics:
+        new_m = m.copy()
+        new_m.table_id = datasource.id
+        logging.info('Importing metric {} from the datasource: {}'.format(
+            new_m.to_json(), i_datasource.full_name))
+        imported_m = i_datasource.metric_class.import_obj(new_m)
+        if (imported_m.metric_name not in
+                [m.metric_name for m in datasource.metrics]):
+            datasource.metrics.append(imported_m)
+
+    for c in i_datasource.columns:
+        new_c = c.copy()
+        new_c.table_id = datasource.id
+        logging.info('Importing column {} from the datasource: {}'.format(
+            new_c.to_json(), i_datasource.full_name))
+        imported_c = i_datasource.column_class.import_obj(new_c)
+        if (imported_c.column_name not in
+                [c.column_name for c in datasource.columns]):
+            datasource.columns.append(imported_c)
+    session.flush()
+    return datasource.id
+
+
+def import_simple_obj(session, i_obj, lookup_obj):
+    make_transient(i_obj)
+    i_obj.id = None
+    i_obj.table = None
+
+    # find if the column was already imported
+    existing_column = lookup_obj(i_obj)
+    i_obj.table = None
+    if existing_column:
+        existing_column.override(i_obj)
+        session.flush()
+        return existing_column
+
+    session.add(i_obj)
+    session.flush()
+    return i_obj