Files in example directories being cached

[karlhorky]

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Reproduction steps:

1. Create a link to an example directory (CodeSandbox A generated)
2. Change a file in that example directory (CodeSandbox A remains)
3. Restart the server
4. CodeSandbox A still remains
5. Clear the Gatsby cache with gatsby clean and restart the server
6. CodeSandbox B generated (from change in 2. above)

Fast solution: I suppose the simplest way of dealing with this for now would be to document the fact that files are cached and that you should clear your Gatsby cache every time you make a change.

Good solution: Clean the cache of example directories on every server restart.