module.callers: Use configuration as source of truth & indicate initialization

[radeksimko]

Background

Currently clients use module.callers command to obtain a list of callers (if any) to help users choose where to run apply or validate since these commands cannot run without prior init.

Additionally clients may also use this list as a list of paths where the user may decide to run init.

Currently module.callers reports initialized module callers (as the data was read from .terraform directory). This is only useful for apply or validate, not so much for init as clients can offer locations which to re-initialize, but not entirely uninitialized directories.

It would still be useful if the client knew which module are initialized however, i.e. in which ones the user can run apply or validate.

Proposal

Update module.callers response, such that we report is_initialized: bool in addition to the URI of the module.

Implementation Notes

Terraform CLI defines "initialized" module as one which

• has all modules installed (as per modules.json manifest) and matching the constraints in the config
• has all providers installed - by comparing the lock file with binaries under .terraform

It may be useful to address the following issues first:

• Support custom plugin cache directory & dev_overrides #24
• terraform/module: Introduce new job to parse provider versions from lock file #992