dotnet-isolated apps with /bin/extensions.json in payloads will not run correctly

We've seen several cases where apps are zipped or built incorrectly and include some older or in-proc artifacts in `/bin`. If we detect an `extensions.json` file in there, we'll use that as our root, which leads to us using this folder as our base for finding extensions and assemblies. This doesn't work in the dotnet-isolated case, where we explicitly build things into the `.azurefunctions` folder.

Ideally we'd:
- log a diagnostic event if we're in some odd state here -- in this case we were starting FunctionsNetHost.exe b/c we never saw anything from the payload to override it (as we never found extensions).
- it also took us a while to detect b/c we don't explicitly log that we've found extensions.json or what the payload of that file was.

For internal reference, ICM number is 463094034.

Start by tracing how the host handles `/bin/extensions.json` and the `.azurefunctions` folder when building the extension and assembly search root. Reproduce the dotnet-isolated payload case and inspect when `FunctionsNetHost.exe` is selected. Done means the odd state produces a diagnostic event and extension discovery logs identify the detected file and its payload.

Written by the indexing model from the issue text.