proto: Remove write_source (pull in `IndexMap` to avoid borrowing issues) #5816

Job	Run time
test on freebsd	4m 34s
test on solaris	6m 25s
msrv	17s
test (macos-latest, stable)	1m 11s
test on illumos	7m 39s
audit	30s
test (windows-latest, stable)	3m 38s
test (ubuntu-latest, beta)	1m 4s
test-android (x86_64-linux-android, x86_64, 25)	3m 34s
lint	49s
test wasm32-unknown-unknown	1m 15s
test-aws-lc-rs	45s
test (ubuntu-latest, stable)	1m 5s
test-android (i686-linux-android, x86, 25)	2m 16s
features (macos-latest)	11m 49s
test-android (x86_64-linux-android, x86_64, 26)	2m 51s
test-android (i686-linux-android, x86, 26)	2m 59s
features (windows-latest)	20m 7s
features (ubuntu-latest)	19m 55s
test on netbsd	5m 59s
	1h 38m 42s

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