Move internal packages into internal folder (#33)

* Move internal packages into internal folder

* Fix testutil imports

stackit/core/core.go

@@ -1,62 +0,0 @@
-package core
-
-import (
-	"context"
-	"fmt"
-	"net/http"
-	"strings"
-
-	"github.com/hashicorp/terraform-plugin-framework/diag"
-	"github.com/hashicorp/terraform-plugin-log/tflog"
-)
-
-// Separator used for concatenation of TF-internal resource ID
-const Separator = ","
-
-type ProviderData struct {
-	RoundTripper                  http.RoundTripper
-	ServiceAccountEmail           string
-	Region                        string
-	DnsCustomEndpoint             string
-	PostgreSQLCustomEndpoint      string
-	PostgresFlexCustomEndpoint    string
-	LogMeCustomEndpoint           string
-	RabbitMQCustomEndpoint        string
-	MariaDBCustomEndpoint         string
-	OpenSearchCustomEndpoint      string
-	RedisCustomEndpoint           string
-	ArgusCustomEndpoint           string
-	SKECustomEndpoint             string
-	ResourceManagerCustomEndpoint string
-}
-
-// DiagsToError Converts TF diagnostics' errors into an error with a human-readable description.
-// If there are no errors, the output is nil
-func DiagsToError(diags diag.Diagnostics) error {
-	if !diags.HasError() {
-		return nil
-	}
-
-	diagsError := diags.Errors()
-	diagsStrings := make([]string, 0)
-	for _, diagnostic := range diagsError {
-		diagsStrings = append(diagsStrings, fmt.Sprintf(
-			"(%s) %s",
-			diagnostic.Summary(),
-			diagnostic.Detail(),
-		))
-	}
-	return fmt.Errorf("%s", strings.Join(diagsStrings, ";"))
-}
-
-// LogAndAddError Logs the error and adds it to the diags
-func LogAndAddError(ctx context.Context, diags *diag.Diagnostics, summary, detail string) {
-	tflog.Error(ctx, summary)
-	diags.AddError(summary, detail)
-}
-
-// LogAndAddWarning Logs the warning and adds it to the diags
-func LogAndAddWarning(ctx context.Context, diags *diag.Diagnostics, summary, detail string) {
-	tflog.Warn(ctx, summary)
-	diags.AddWarning(summary, detail)
-}