The imports will be replaced with the contents of the file in `models/gdp_over_time.squiggle` upon compilation. The `.squiggleU` file will be converted into a `.squiggle` file with the `import` statement having this replacement.
Watches `.squiggleU` files in the current directory (and subdirectories) and rebuilds them when they are saved. Note that this will _not_ rebuild files when their dependencies are changed, just when they are changed directly.