Validate providers in expanding modules. Expanding modules cannot have provider configurations with non-empty configs, which includes having a version configured. If an empty or alias-only block is passed, the provider must be passed through the providers argument on the module call
Previously, adding a version constraint to a module that was previously
recorded without a version in the module manifest would cause a panic.
Instead, we now use a slight variant of the "dependencies have changed"
error that doesn't try to print out a specific version number.
Previously the behavior for loading and installing modules was included in
the same package as the representation of the module tree (in the
config/module package).
In our new world, the model of a module tree (now called a "Config") is
included in "configs" along with the Module and File structs. This new
package replaces the loading and installation functionality previously
in config/module with new equivalents that work with the model objects
in "configs".
As of this commit, only the loading functionality is implemented. The
installation functionality will follow in subsequent commits.
Pam Selle