cmd/go: fix go get with ... fails in module mode

[hxzhao527]

There is a mistake about haveGoFiles detection in modload, and it caused go get with ... fails in module mode.

Fixes #29363

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Patch Set 1:

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Message from Brad Fitzpatrick:

Patch Set 3:

Could you add a test?

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Message from 浩翔 赵:

Patch Set 3:

Patch Set 3:

Could you add a test?

emmm, I don't know how to append a test to import_test.go.
I just test this by the go get command.
Let me find a way to test this properly. Maybe need some moments.

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Message from 浩翔 赵:

Patch Set 4:

Test added.

I can't access vcs-test.golang.org/git/querytest.git, so I just use github.com/google/codesearch as the test target.

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[hxzhao527]

/comments off

[hxzhao527]

@bcmills Hi here. I'm sorry, but the Gerrit is too difficult for me to send a comment.
I doubt about why we need a fix for the general case.

Now go get in mod mode is not always failed, just when the package not in module dependencies.
As the #27215 shows, we can call go get golang.org/x/perf/cmd/benchstat and then go get golang.org/x/perf/cmd/..., even go get golang.org/x/perf/...cmd/....

After the first go get add the package to module dependencies, everything works.
So the first call is the just one needed to fix. Is this correct?