chore(gatsby-transformer-sharp): remove fragment copying from plugins

[tmaximini]

Description

Since #15284, fragments no longer need to be copied over to the .cache directory because gatsby looks through node_modules

This PR removes that copying mechanism from gatsby-transformer-sharp and gatsby-source-contentful and updates its peer dependencies to "gatsby": "^2.12.1"

[sidharthachatterjee]

Thanks for doing this, Thomas! It was great pairing with you 💚

[sidharthachatterjee]

🥇

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