Add a linter for clashing normalized field names

[istreeter]

A schema might contain two fields with names like exampleField and example_field i.e. both snake case and camel case.

When we normalize field names for BigQuery those two fields both get normalized to the same example_field, and this creates problems for loading the data.

There should be a linter, so that if a schema would contain clashing field names after normalization then it produces a warning.