Improve pallet purchase file structure (paritytech#6780)

Linked to issue paritytech#590 
I moved the mod, tests, mock and benchmarking to their own seperate file
to reduce the bloat inside purchase.rs

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Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>