fix: purchase receipt gl entries with same item code

[nextchamp-saqib]

Steps to replicate:

• Enable perpetual inventory for service items
• Create a Purchase Receipt with 2 service items having same item code but different rates
• Create Purchase Invoice for that receipt & check the ledger, there will be an invalid entry against COGS (item's expense account)

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This happens since the GL Entries made for the item in purchase receipt is merged into one regardless of voucher_detail_no.
This is a problem since voucher_detail_no is used to check if the expense is booked in the purchase receipt (line 621: purchase_invoice.py)
Since the GL Entries are merged into one into voucher_detail_no, the expense is booked again for 2nd item in purchase invoice.

[coveralls]

Coverage decreased (-0.01%) to 42.679% when pulling cceabff on nextchamp-saqib:pr-item-gl-fix into 6894cc2 on frappe:develop.