$ cd a
$ go build # success
$ cd ../b
$ go build # success
$ cd ../c
$ time go build -v
go: alltom.com/b@v0.0.0-00010101000000-000000000000 requires
alltom.com/a@v0.0.0-00010101000000-000000000000: unrecognized import path "alltom.com/a" (https fetch: Get https://alltom.com/a?go-get=1: dial tcp 52.216.205.138:443: i/o timeout)
real 0m30.281s
user 0m0.075s
sys 0m0.036s