Log a warning when a command is not found or a module is not found

[AnatoliB]

Resolves #491

When a command is invoked from the function code, but this command cannot be found in any available module, logs the following warning:

The app may be missing a module containing the '{0}' command definition. If this command belongs to a module available on the PowerShell Gallery, add a reference to this module to requirements.psd1. Make sure this module is compatible with PowerShell 7. For more details, see https://aka.ms/functions-powershell-managed-dependency.

When Import-Module is invoked from the function code but this module is not available, logs the following warning:

The app may be missing the '{0}' module. If '{0}' is available on the PowerShell Gallery, add a reference to this module to requirements.psd1. Make sure this module is compatible with PowerShell 7. For more details, see https://aka.ms/functions-powershell-managed-dependency.

/cc @eamonoreilly @stefanushinardi for the warning message wording.

[Francisco-Gamino]

One minor comment. Otherwise, LGTM