fix(gatsby): notify when Gatsby cache is incomplete

[vladar]

Description

This PR adds a warning and wipes the .cache when the public folder is missing but the .cache folder exists.

Why

Builds can break if someone deletes the public folder but not the .cache (with cryptic errors about missing page-data files).

There is one case when doing this still leads to accidentally working builds: when the project only has pages without any queries. This behavior actually relies on our "bug" - at the moment we always re-run page queries without dependencies (or in other words - pages without graphql queries in them).

This skipped test sums up the problem:

gatsby/packages/gatsby/src/query/__tests__/data-tracking.js

Lines 1241 to 1243 in ecc0ad3

	// TO-DO: this is known issue - we always rerun queries for pages with no dependencies
	// this mean that we will retry to rerun them every time we restart gatsby
	it.skip(`rerunning should not run any queries (with restart)`, async () => {

That's exactly what happens in gatsby-admin and our long-term-caching integration tests (we don't have any build-time graphql queries in those and delete the public folder without deleting the .cache). At the moment this "works" but won't work anymore after #27504 lands (which actually fixes this bug).

[wardpeet]

LGTM :) this has created issues in the past. I'm sure of it. THank you!

[KyleAMathews]

🙏 this has frustrated a lot of people in the past, thanks!

[quantizor]

I'm a bit disappointed that y'all are doubling down on requiring public instead of keeping the page-data files in .cache and copying them into public during builds. This is an anti-pattern.

[KyleAMathews]

@probablyup this is something we can do in the future but it's a bigger change than simply verifying the state of the public folder.