Support bundled gems like pstore and ostruct #378

Job	Run time
ruby-versions / Generate Ruby versions	3s
build (2.5, 0.1.7, /tmp/local)	51s
build (2.5, 0.1.7)	34s
build (2.5, 0.2.5, /tmp/local)	54s
build (2.5, 0.2.5)	34s
build (2.6, 0.1.7, /tmp/local)	59s
build (2.6, 0.1.7)	35s
build (2.6, 0.2.5, /tmp/local)	1m 0s
build (2.6, 0.2.5)	36s
build (2.7, 0.1.7, /tmp/local)	58s
build (2.7, 0.1.7)	33s
build (2.7, 0.2.5, /tmp/local)	59s
build (2.7, 0.2.5)	34s
build (3.0, 0.1.7, /tmp/local)	28s
build (3.0, 0.1.7)	21s
build (3.0, 0.2.5, /tmp/local)	28s
build (3.0, 0.2.5)	24s
build (3.1, 0.1.7, /tmp/local)	28s
build (3.1, 0.1.7)	20s
build (3.1, 0.2.5, /tmp/local)	27s
build (3.1, 0.2.5)	20s
build (3.2, 0.1.7, /tmp/local)	32s
build (3.2, 0.1.7)	26s
build (3.2, 0.2.5, /tmp/local)	29s
build (3.2, 0.2.5)	26s
build (3.3, 0.1.7, /tmp/local)	29s
build (3.3, 0.1.7)	23s
build (3.3, 0.2.5, /tmp/local)	35s
build (3.3, 0.2.5)	20s
build (3.4, 0.1.7, /tmp/local)	32s
build (3.4, 0.1.7)	21s
build (3.4, 0.2.5, /tmp/local)	28s
build (3.4, 0.2.5)	23s
build (head, 0.1.7, /tmp/local)	25s
build (head, 0.1.7)	20s
build (head, 0.2.5, /tmp/local)	28s
build (head, 0.2.5)	23s
	19m 26s

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