* configs/configupgrade: detect possible relative module sources
If a module source appears to be a relative local path but does not have
a preceding ./, print a #TODO message for the user.
* internal/initwd: limit go-getter detectors to those supported by terraform
* internal/initwd: move isMaybeRelativeLocalPath check into getWithGoGetter
To avoid making two calls to getter.Detect, which potentially makes
non-trivial API calls, the "isMaybeRelativeLocalPath" check was moved to
a later step and a custom error type was added so user-friendly
diagnostics could be displayed in the event that a possible relative local
path was detected.
Identify module sources that look like relative paths ("child" instead
of "./child", for example) and surface a helpful error.
Previously, such module sources would be passed to go-getter, which
would fail because it was expecting an absolute, or properly relative,
path. This commit moves the check for improper relative paths sooner so
a user-friendly error can be displayed.
