Handle / in lambda handlers

[Czechh]

What does this PR do?

While using the serverless datadog plugin if a handler is inside of a folder then we get the following error:

[ERROR] Runtime.ImportModuleError: Unable to import module 'datadog_lambda.handler': No module named 'lambda/handler' |  
-- | --

To fix this we simply need to make sure we replace / for . before loading it in the layer handler.

Motivation

Started from this issue DataDog/serverless-plugin-datadog#44

Testing Guidelines

• Integration tests work
• Also verified with a manual test using a handler with . instead of / and it works great.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[tianchu]

@Czechh mind adding some unit tests?

[Czechh]

@tianchu added units for the helper. Good call