[release-branch.go1.17] [tailscale1.17] go/build: ignore package main files in TestDependencies

The tree has package main files scattered around
in it for the purposes of running go generate.

They're all marked "// +build ignore",
which gets special handling in TestDependencies.
It would be nice to be able to use other build tags,
such as "generate", as suggested by the go generate
design doc. Plus the build tag syntax is changing.

This change skips all "package main" files.
By definition these aren't importable,
so they can't contribute to the dependency tree.

We can't quite eliminate the "// +build ignore"
check, as it is used by packages runtime and syscall.
But it's still a step in the right direction.

Change-Id: Ib9449acfdba75f570b87a4200afe944910d76222
Reviewed-on: https://go-review.googlesource.com/c/go/+/339592
Trust: Josh Bleecher Snyder <[email protected]>
Run-TryBot: Josh Bleecher Snyder <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

(cherry picked from https://golang.org/cl/339592)

Author: josharian

src/go/build/deps_test.go

@@ -652,6 +652,9 @@ func findImports(pkg string) ([]string, error) {
 		if err != nil {
 			return nil, fmt.Errorf("reading %v: %v", name, err)
 		}
+		if info.parsed.Name.Name == "main" {
+			continue
+		}
 		if bytes.Contains(info.header, buildIgnore) {
 			continue
 		}