Copy fragments from build files to ease up developing

[m-allanson]

Description

When changing Gatsby's GraphQL fragments and testing these changes using gatsby-dev-cli, the changed fragments need to be manually copied from Gatsby's src directories. This should be done automatically.

Note that this is a fairly rare bug that can be worked around. It'd be nice to fix but it's not essential as it only affects people editing Gatsby's built in GraphQL fragments.

Refs: #5698
Refs: #5659 (comment)

Steps to reproduce

• Set up a Gatsby dev environment
• Change the GraphQL fragments in a Gatsby plugin, e.g. gatsby-transformer-sharp/src/fragments.js
• Run gatsby-dev to use your modified Gatsby on a test site
• Note that the fragments have not been updated in your test site

Steps to fix manually

• Manually copy from /projects/gatsby-repo/packages/gatsby-transformer-sharp/src/fragments.js to /projects/gatsby-test-site/node_modules/gatsby-transformer-sharp/src/fragments.js
• Delete your site's .cache directory
• Run gatsby develop and note that the correct fragments are now being used

Expected result

The fragments should be updated automatically.

Actual result

The fragments have to be updated manually.

[KyleAMathews]

This would be nice but probably there's higher priorities. Closing until it comes up again.

[axe312ger]

😿