Only resolve ligatures externals in commonjs environment #920

Job	Run time
lint	3m 24s
build	1m 25s
test-integration (18, ubuntu, chromium)	4m 1s
test-integration (18, ubuntu, firefox)	4m 6s
test-unit (18, ubuntu)	1m 58s
test-integration (18, ubuntu, webkit)	3m 21s
test-unit (18, macos)	4m 20s
test-unit (18, windows)	6m 33s
test-unit-coverage	1m 4s
release-dry-run (18)	1m 59s
	32m 11s

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