This commit implements most of the intended functionality of the upgrade
command for rewriting configurations.
For a given module, it makes a list of all providers in use. Then it
attempts to detect the source address for providers without an explicit
source.
Once this step is complete, the tool rewrites the relevant configuration
files. This results in a single "required_providers" block for the
module, with a source for each provider.
Any providers for which the source cannot be detected (for example,
unofficial providers) will need a source to be defined by the user. The
tool writes an explanatory comment to the configuration to help with
this.
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