Redesign `spirv-builder`'s watch API #1178

Job	Run time
Test (windows-2022)	19m 23s
Test (macOS-latest)	12m 38s
Compiletest (macOS-latest)	11m 7s
Test (ubuntu-24.04)	11m 4s
cargo-deny	1m 15s
Compiletest (ubuntu-24.04)	7m 50s
Compiletest (windows-2022)	15m 45s
Difftest (windows-2022)	24m 29s
Android (ubuntu-24.04, aarch64-linux-android)	8m 32s
Difftest (macOS-latest)	17m 17s
Lint	5m 48s
Difftest (ubuntu-24.04)	13m 49s
test_success	2s
	2h 28m 59s

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