Correctly emit warnings for unnecessary PostCSS plugins in package.json

[mischnic]

When declaring the postcss config in package.json, the json paths for the diagnostic need to be prefixed (by the "postcss" packageKey argument to loadConfig).

Closes #7984

[parcel-benchmark]

Benchmark Results

Kitchen Sink 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

No bundles found, this is probably a failed build...

Cached Bundles

No bundles found, this is probably a failed build...

React HackerNews ✅

Timings

Description	Time	Difference
Cold	8.82s	-70.00ms
Cached	437.00ms	-0.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

AtlasKit Editor ✅

Timings

Description	Time	Difference
Cold	1.51m	+255.00ms
Cached	2.41s	-27.00ms

Cold Bundles

Bundle	Size	Difference	Time	Difference
dist/index.9eefec02.js	3.32mb	+144.00b ⚠️	29.57s	+89.00ms
dist/esm.6190c9ac.js	61.54kb	+0.00b	1.14m	+31.07s ⚠️
dist/workerHasher.e50d242f.js	1.72kb	+0.00b	1.14m	+31.07s ⚠️
dist/16.1969624f.js	1.08kb	+0.00b	36.64s	+3.21s ⚠️
dist/16.069344b7.js	905.00b	+0.00b	36.64s	+3.21s ⚠️
dist/simpleHasher.46d6f2e5.js	742.00b	+0.00b	1.14m	+31.07s ⚠️
dist/index.html	240.00b	+0.00b	1.14m	+38.65s ⚠️

Cached Bundles

Bundle	Size	Difference	Time	Difference
dist/index.2009fd6f.js	3.32mb	+144.00b ⚠️	29.36s	-114.00ms
dist/esm.6190c9ac.js	61.54kb	+0.00b	1.14m	+31.28s ⚠️
dist/workerHasher.e50d242f.js	1.72kb	+0.00b	1.14m	+31.28s ⚠️
dist/16.1969624f.js	1.08kb	+0.00b	36.78s	+3.30s ⚠️
dist/index.html	240.00b	+0.00b	1.14m	+35.92s ⚠️

Three.js ✅

Timings

Description	Time	Difference
Cold	6.36s	+29.00ms
Cached	259.00ms	-2.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

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