Raise a warning for required, but uninstalled packages

Describe the feature

The error message for an uninstalled package comes straight out of the Jinja library:

Compilation Error in analysis scratch (analysis/scratch.sql)
    'dbt_utils' is undefined

This error message is strictly true, but I think it might be worth taking the time to write a more explicit warning message for uninstalled packages

Warning: The following packages are listed as required in `packages.yml` but have not been installed
- dbt_utils

Run `dbt deps` to install these packages

Since we have the expected packages in packages.yml and the installed packages in dbt_modules, this should be possible, right? And not that hard?

Note: I’ve only got it as a warning so that the places where the package is referenced still causes an error, but I’m open to having this as an error instead

Describe alternatives you’ve considered

Keep as is

Who will this benefit?

Confused users

Are you interested in contributing this feature?

No ty 😃

[jtcohen6]

@jaypeedevlin I don’t know why I thought this would be so tricky! At the prompting of your PR comment, I took a look down this code path, and I think it could be as simple as sticking a check right in load_dependencies: https://github.com/fishtown-analytics/dbt/blob/1f33b6a74a614aa346824f16c5629e4d910d2106/core/dbt/config/runtime.py#L327-L334

    def load_dependencies(self) -> Mapping[str, 'RuntimeConfig']:
        if self.dependencies is None:
            all_projects = {self.project_name: self}
            internal_packages = get_include_paths(self.credentials.type)
            # raise exception if fewer installed packages than specified packages
            package_dirs = self._get_project_directories()
            count_packages_specified = len(self.packages.packages)
            count_packages_installed = len(tuple(package_dirs))
            if count_packages_specified > count_packages_installed:
                raise_compiler_error(
                    f'dbt found {count_packages_specified} package(s) specified in packages.yml, '
                    f'but only {count_packages_installed} package(s) installed in {self.modules_path}. '
                    f'Run "dbt deps" to install package dependencies.'
                )
            project_paths = itertools.chain(internal_packages, package_dirs)

We can’t match on names, since self.packages.packages is just the info from packages.yml (which may or may not match the package name, if it’s a git repo name or local directory). We also can’t check lengths for perfect equality, since it’s possible that count_packages_installed will be greater than count_packages_installed, if the specified packages have package dependencies of their own. But I’d say this is a lot better than nothing, and I’d be excited about pairing it with the kinder UndefinedMacroException you’ve got over in #3343.

[jaypeedevlin]

@jtcohen6 I’d love to jump onto this one in the next day or so.

Just to clarify, regarding:

I’m going to mark this a good first issue! And a nice chance to add a new dbt exception 😃

Will catching and reraising the exception be enough, or do I also need to modify exceptions.py?