Do not warn for internally loaded components

[balloob]

Description:

In #21070 I made a mistake by using the externally facing get_component inside get_platform, causing a warning to be printed for errors that were expected.

This fixes it. It also fixes the custom component warning.

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

Example entry for configuration.yaml (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[SukramJ]

I think this PR causes a lint in dev:
homeassistant/loader.py:163:8: W0612: Unused variable 'index' (unused-variable)

Thats why my PR build #21241 fails in travis. Others are affected too.