Fix eager typecheck of `Map` #747

Job	Run time
gradle-check	9m 48s
upload-event-file	7s
check-pkl-github-actions	10s
pkl-doc-linux-amd64-snapshot	0s
gradle-check-windows	0s
pkl-cli-macOS-amd64-snapshot	0s
pkl-doc-alpine-linux-amd64-snapshot	0s
pkl-doc-macOS-aarch64-snapshot	0s
pkl-cli-linux-aarch64-snapshot	0s
pkl-cli-linux-amd64-snapshot	0s
pkl-doc-windows-amd64-snapshot	0s
pkl-doc-macOS-amd64-snapshot	0s
pkl-cli-macOS-aarch64-snapshot	0s
pkl-doc-linux-aarch64-snapshot	0s
pkl-cli-windows-amd64-snapshot	0s
pkl-cli-alpine-linux-amd64-snapshot	0s
verdict	2s
	10m 7s

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