perf: speed up customer import (#47738)

* perf: Avoid fetching customer from DB

This is likely code from bygone era where is_new and doc_before_save didn't exist?

I see no reason to do it in this weird manner. This code still appears
to be wrong but I'll leave it as is for now.

* fix: don't validate internal customer for non-internal customer

This was likely missed out while adding validations

erpnext/selling/doctype/customer/customer.py

@@ -152,8 +152,7 @@ class Customer(TransactionBase):
 		self.validate_currency_for_receivable_payable_and_advance_account()
 
 		# set loyalty program tier
-		if frappe.db.exists("Customer", self.name):
-			customer = frappe.get_doc("Customer", self.name)
+		if not self.is_new() and (customer := self.get_doc_before_save()):
 			if self.loyalty_program == customer.loyalty_program and not self.loyalty_program_tier:
 				self.loyalty_program_tier = customer.loyalty_program_tier
 
@@ -207,6 +206,7 @@ class Customer(TransactionBase):
 	def validate_internal_customer(self):
 		if not self.is_internal_customer:
 			self.represents_company = ""
+			return
 
 		internal_customer = frappe.db.get_value(
 			"Customer",