**Apache Airflow version**: 1.10.12


**Environment**:

- **Cloud provider or hardware configuration**: Google Cloud, custom-8-16384 
- **OS** (e.g. from /etc/os-release): CentOS Linux release 7.7.1908 (Core)
- **Kernel** (e.g. `uname -a`): 3.10.0-1062.18.1.el7.x86_64 #1 SMP Tue Mar 17 23:49:17 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

**What happened**:

Class method [on_load](https://github.com/apache/airflow/blob/ce66bc944d246aa3b51cce6e2fc13cd25da08d6e/airflow/plugins_manager.py#L102) of AirflowPlugin class doesn't run after plugins located in plugin folder, only for plugins installed through pip.

**What you expected to happen**:

Method 'on_load' should run on plugins initialization, that are located in plugin folder

**How to reproduce it**:

1) Create plugin with next code and place it in plugins folder:

```
class TestPlugin(AirflowPlugin):
    menu_links = [MenuLink(category='Google', name='Google', url='google.com')]

    @classmethod
    def on_load(cls, *args, **kwargs):
        raise AirflowPluginException("Test On Load Exception")
```
2) Reload webserver.

_Result_: Webserver logs are empty, new menu appeared in UI.
_Expected result_: Webserver logs should contain information with raised exception, new menu shouldn't appear in UI.
_Reason_: on_load method is called only for airflow.plugins.* entry_points
https://github.com/apache/airflow/blob/ce66bc944d246aa3b51cce6e2fc13cd25da08d6e/airflow/plugins_manager.py#L152

**Additional information**: It is not clear what is the reason for skipping this method call for plugins contained in the folder. At the same time, plugin validation successfully run for plugins from a folder and installed via a package. Validation uses validate method in same AirflowPlugin class and can be extended by user.
https://github.com/apache/airflow/blob/b9dc3c51ba2cba1c61d327488cecf2623d6445b3/airflow/plugins_manager.py#L96
tests/plugins/test_plugin.py
#
#
#
from flask import Blueprint
from flask_appbuilder import BaseView as AppBuilderBaseView, expose
from airflow.executors.base_executor import BaseExecutor
from airflow.hooks.base import BaseHook
from airflow.models.baseoperator import BaseOperator
from airflow.plugins_manager import AirflowPlugin
from airflow.sensors.base import BaseSensorOperator
from tests.test_utils.mock_operators import (
    AirflowLink,
    AirflowLink2,
    CustomBaseIndexOpLink,
    CustomOpLink,
    GithubLink,
    GoogleLink,
)
class PluginHook(BaseHook):
    pass
class PluginOperator(BaseOperator):
    pass
class PluginSensorOperator(BaseSensorOperator):
    pass
class PluginExecutor(BaseExecutor):