bb: don't print module conflict error duplicates

[hugelgupf]

This desperately needs some tests too.

before:

./u-root -defaultsh="" ./cmds/core/ip ./cmds/core/init ~/cpu/cmds/cpu
2021/12/29 21:33:30 Disabling CGO for u-root...
2021/12/29 21:33:30 Build environment: GOARCH=amd64 GOOS=linux GOROOT=/usr/lib/google-golang GOPATH=/usr/local/google/home/chrisko/go CGO_ENABLED=0
2021/12/29 21:33:30 NOTE: building with the new gobusybox; to get old behavior, use -build=bb

2021/12/29 21:33:32 Conflicting module dependencies on github.com/u-root/u-root:
2021/12/29 21:33:32   github.com/u-root/cpu uses version v0.8.0
2021/12/29 21:33:32   your request to compile github.com/u-root/u-root from /usr/local/google/home/chrisko/go/src/github.com/u-root/u-root uses version 

2021/12/29 21:33:32 Suggestion to resolve: add `replace github.com/u-root/u-root => ../go/src/github.com/u-root/u-root` to /usr/local/google/home/chrisko/cpu/go.mod


2021/12/29 21:33:32 Conflicting module dependencies on github.com/u-root/u-root:
2021/12/29 21:33:32   github.com/u-root/cpu uses version v0.8.0
2021/12/29 21:33:32   your request to compile github.com/u-root/u-root from /usr/local/google/home/chrisko/go/src/github.com/u-root/u-root uses version 

2021/12/29 21:33:32 Suggestion to resolve: add `replace github.com/u-root/u-root => ../go/src/github.com/u-root/u-root` to /usr/local/google/home/chrisko/cpu/go.mod

2021/12/29 21:33:32 Gobusybox error: collecting and putting dependencies in place failed: conflicting module dependencies found
2021/12/29 21:33:32 Build error: error building: preserving bb generated source directory at /tmp/u-root85691852/builder592929704 due to error

after

./u-root -defaultsh="" ./cmds/core/ip ./cmds/core/init ~/cpu/cmds/cpu
2021/12/29 21:31:22 Disabling CGO for u-root...
2021/12/29 21:31:22 Build environment: GOARCH=amd64 GOOS=linux GOROOT=/usr/lib/google-golang GOPATH=/usr/local/google/home/chrisko/go CGO_ENABLED=0
2021/12/29 21:31:22 NOTE: building with the new gobusybox; to get old behavior, use -build=bb

2021/12/29 21:31:25 Conflicting module dependencies on github.com/u-root/u-root:
2021/12/29 21:31:25   module github.com/u-root/cpu depends on it @ version v0.8.0
2021/12/29 21:31:25   your request to compile github.com/u-root/u-root/cmds/core/ip depends on it @ directory /usr/local/google/home/chrisko/go/src/github.com/u-root/u-root

2021/12/29 21:31:25 Suggestion to resolve: add `replace github.com/u-root/u-root => ../go/src/github.com/u-root/u-root` to /usr/local/google/home/chrisko/cpu/go.mod

2021/12/29 21:31:25 Gobusybox error: collecting and putting dependencies in place failed: conflicting module dependencies found
2021/12/29 21:31:25 Build error: error building: preserving bb generated source directory at /tmp/u-root4014131519/builder2173858192 due to error