Add support for WHERE with imported symbols

[sergisiso]

WHEREs with imported symbols create a CodeBlock with:
PSyclone doesn't yet support reference to imported symbols inside WHERE clauses.

Now that we can resolve modules in the frontend before iterating down to the execution section we can potentially fix this. Or improve the message to provide the instructions to fix it.

[RobWatersMet]

Just to second this as we have a number of these in UKCA.

For example:

https://code.metoffice.gov.uk/trac/ukca/browser/main/trunk/src/science/core/chemistry/asad/asad_hetero.F90#L283
https://code.metoffice.gov.uk/trac/ukca/browser/main/trunk/src/science/core/chemistry/asad/asad_hetero.F90#L322