VLOOKUP is incompatible with change imported files

[aarondr77]

Data to reproduce: fund_info.csv performance-Copy1.csv performance.csv

1. Import performance and fund_info
2. Write a vlookup in the fund_info dataset that references a column in performance
3. Change imported data to replace performance with perofmrnace-Copy
4. Get an error

This is likely because VLOOKUP makes set column formula dependent on dataframe names in a way no other steps are (since its the only one that is cross sheet)

To resolve it we could store a map from original sheet name to sheet index so when we re-parse the formula we can update it.