remove "main" package.json entries and build step

[blakef]

Summary: Conform to the style of other packages that are built using the monorepo build script, which don't use the "main" field. Instead these utilise:

• root level index.flow.js → src/
• .flowconfig (both github and other) to resolve the module.

Changelog: [Internal]

Differential Revision: D56879279

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D56879279

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D56879279

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	19,511,397	-7
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	22,883,955	+0
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: 5a120322bfe442412f14485663f6f63fc5af3e0a Branch: main