healthd crash: ImportError: No module named sonic_platform.platform

[vboykox]

Description
Receiving next log in /var/log/syslog

Traceback (most recent call last):
  File "/usr/local/bin/healthd", line 108, in <module>
    main()
  File "/usr/local/bin/healthd", line 104, in main
    health_monitor.run()
  File "/usr/local/bin/healthd", line 71, in run
    import sonic_platform.platform
ImportError: No module named sonic_platform.platform

Steps to reproduce the issue:

1. Deploy the image after [system-health] Add support for monitoring system health #4835 merge
2. see /var/log/syslog

Describe the results you received:
the heathd monitor does not crash

Describe the results you expected:
The daemon is started inside PMON container or the sonic_platform package is installed to the host fs.

Additional information you deem important (e.g. issue happens only occasionally):

SONiC Software Version: SONiC.master.5-dirty-20201013.025725
Distribution: Debian 10.6
Kernel: 4.19.0-9-2-amd64
Build commit: bba5df05
Build date: Tue Oct 13 07:09:25 UTC 2020
Built by: johnar@jenkins-worker-8

Introduced by:
#4835

[vboykox]

@Junchao-Mellanox

[Junchao-Mellanox]

Hi, this feature requred to load platform API on host side, please see design document: https://github.com/Azure/SONiC/blob/master/doc/system_health_monitoring/system-health-HLD.md#4-platform-api-and-pmon-related-change-to-support-this-new-service

And during our test on Mellanox platform, it works.