Rename `ext-src` to `src` in SDK #2587

Job	Run time
Cancel redundant actions already in progress	3s
Build Wasp (ubuntu-20.04, latest, 8.10.7, 3.6.2.0)	9m 22s
Build Wasp (macos-latest, latest, 8.10.7, 3.6.2.0)	15m 2s
Build Wasp (windows-latest, latest, 8.10.7, 3.6.2.0)	9m 39s
Build Wasp (ubuntu-20.04, 18, 8.10.7, 3.6.2.0)	9m 39s
Build Wasp (ubuntu-20.04, 20, 8.10.7, 3.6.2.0)	11m 4s
	54m 49s

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