Only allow builds when go build also works

[hugelgupf]

This PR adds support for Go workspaces, including vendored workspaces, as well as support for offline builds in all cases -- vendored workspaces, vendored command modules, and GOPATH.

This removes support for "synthesized" go.mod files, in which stuff like

git clone https://github.com/u-root/u-root
git clone https://github.com/u-root/cpu
makebb ./u-root/cmds/core/init ./cpu/cmds/cpud

used to work. In the same situation, go build ./u-root/cmds/core/init ./cpu/cmds/cpud would not have worked.

makebb ./u-root/cmds/core/init ./cpu/cmds/cpud
...
Preserving bb generated source directory at /tmp/bb-3596733969 due to error: finding packages failed: err: exit status 1: stderr: go: go.mod file not found in current directory or any parent directory; see 'go help modules'
: gobusybox has removed previous multi-module functionality in favor of Go workspaces -- read https://github.com/u-root/gobusybox#path-resolution--multi-module-builds for more

go build doesn't work in the same situation. We now require that go build work.

For the same situation, Go workspaces exist:

mkdir workspace
cd workspace
git clone https://github.com/u-root/u-root
git clone https://github.com/u-root/cpu
go work init ./u-root
go work use ./cpu
makebb ...

For more information, read the new README.

Closes #109
Closes #35
Closes #64
Closes #24
Closes #14
Closes #8
Updates #104.

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