core: Reject provider schemas with version < 0

There's no reason for a negative version, so by blocking it now we'll
ensure that none creep in.

The more practical short-term motivation for this is that we're still
using uint64 for these internally in some cases and so this restriction
ensures that we won't run into rough edges when converting from int64 to
uint64 at those boundaries until we later fix everything to use int64
consistently.

terraform/schemas.go

@@ -131,13 +131,33 @@ func loadProviderSchemas(schemas map[string]*ProviderSchema, config *configs.Con
 			ResourceTypeSchemaVersions: make(map[string]uint64),
 		}
 
+		if resp.Provider.Version < 0 {
+			// We're not using the version numbers here yet, but we'll check
+			// for validity anyway in case we start using them in future.
+			diags = diags.Append(
+				fmt.Errorf("invalid negative schema version provider configuration for provider %q", typeName),
+			)
+		}
+
 		for t, r := range resp.ResourceTypes {
 			s.ResourceTypes[t] = r.Block
 			s.ResourceTypeSchemaVersions[t] = r.Version
+			if r.Version < 0 {
+				diags = diags.Append(
+					fmt.Errorf("invalid negative schema version for resource type %s in provider %q", t, typeName),
+				)
+			}
 		}
 
 		for t, d := range resp.DataSources {
 			s.DataSources[t] = d.Block
+			if d.Version < 0 {
+				// We're not using the version numbers here yet, but we'll check
+				// for validity anyway in case we start using them in future.
+				diags = diags.Append(
+					fmt.Errorf("invalid negative schema version for data source %s in provider %q", t, typeName),
+				)
+			}
 		}
 
 		schemas[typeName] = s