Linter Core: change lots of behaviour

* Use the Django migration classes to manipulate these objects
this includes the discovery of migrations through the MigrationLoader
(instead of manually walking through the Django project and find the migrations through a regex)
and representing a Migration through the Django class.
* Get the SQL of a migration through a 'call_command' call of 'sqlmigrate'
That speeds up greatly the linter since we don't have to setup Django for each migration
* Add the database to the cache path

django_migration_linter/utils.py

@@ -15,36 +15,7 @@
 from __future__ import print_function
 
 import os
-import sys
-
-from .constants import MIGRATION_FOLDER_NAME
-
-
-def is_django_project(path):
-    django_manage_file = os.path.join(path, "manage.py")
-    return os.path.isfile(django_manage_file)
-
-
-def is_directory(path):
-    return os.path.isdir(path)
-
-
-def print_stderr(*args, **kwargs):
-    print(*args, file=sys.stderr, **kwargs)
-
-
-def find_project_settings_module(path):
-    """Explore path recursively to the first settings.py file
-    and translate the name to package notation 'mysite.settings'
-    """
-    for root, dirs, files in os.walk(path):
-        for file_name in files:
-            if file_name == "settings.py":
-                return (
-                    os.path.join(root.replace(path, ""), file_name)
-                    .replace(os.sep, ".")
-                    .rstrip(".py")
-                )
+from importlib import import_module
 
 
 def split_path(path):
@@ -67,17 +38,26 @@ def split_path(path):
 
 
 def split_migration_path(migration_path):
+    from django.db.migrations.loader import MIGRATIONS_MODULE_NAME
+
     decomposed_path = split_path(migration_path)
     for i, p in enumerate(decomposed_path):
-        if p == MIGRATION_FOLDER_NAME:
+        if p == MIGRATIONS_MODULE_NAME:
             return decomposed_path[i - 1], os.path.splitext(decomposed_path[i + 1])[0]
 
 
-def compose_migration_path(django_folder, app_name, migration):
-    return os.path.join(
-        django_folder, app_name, MIGRATION_FOLDER_NAME, "{0}.py".format(migration)
-    )
-
-
 def clean_bytes_to_str(byte_input):
     return byte_input.decode("utf-8").strip()
+
+
+def get_migration_abspath(app_label, migration_name):
+    from django.db.migrations.loader import MigrationLoader
+
+    module_name, _ = MigrationLoader.migrations_module(app_label)
+    migration_path = "{}.{}".format(module_name, migration_name)
+    migration_module = import_module(migration_path)
+
+    migration_file = migration_module.__file__
+    if migration_file.endswith(".pyc"):
+        migration_file = migration_file[:-1]
+    return migration_file