cmd/go/internal/imports: match solaris files for illumos

Like the android/linux pair, match files with GOOS=solaris if
GOOS=illumos

Follow-up for CL 174457

Fixes #32370
Updates #20603

Change-Id: Id01de8bb3990d7bbb22f735a4abed2b3cf8b4062
Reviewed-on: https://go-review.googlesource.com/c/go/+/179997
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/imports/build.go

@@ -138,6 +138,9 @@ func matchTag(name string, tags map[string]bool, want bool) bool {
 	if name == "linux" {
 		have = have || tags["android"]
 	}
+	if name == "solaris" {
+		have = have || tags["illumos"]
+	}
 	return have == want
 }
 
@@ -152,7 +155,9 @@ func matchTag(name string, tags map[string]bool, want bool) bool {
 //     name_$(GOARCH)_test.*
 //     name_$(GOOS)_$(GOARCH)_test.*
 //
-// An exception: if GOOS=android, then files with GOOS=linux are also matched.
+// Exceptions:
+//     if GOOS=android, then files with GOOS=linux are also matched.
+//     if GOOS=illumos, then files with GOOS=solaris are also matched.
 //
 // If tags["*"] is true, then MatchFile will consider all possible
 // GOOS and GOARCH to be available and will consequently

src/cmd/go/internal/imports/testdata/illumos/a_illumos.go

+package illumos
+
+import _ "a"

src/cmd/go/internal/imports/testdata/illumos/b_illumos_amd64.go

+package illumos
+
+import _ "b"

src/cmd/go/internal/imports/testdata/illumos/c_solaris.go

+package illumos
+
+import _ "c"

src/cmd/go/internal/imports/testdata/illumos/d_solaris_amd64.go

+package illumos
+
+import _ "d"

src/cmd/go/internal/imports/testdata/illumos/e.go

+// +build illumos
+
+package illumos
+
+import _ "e"

src/cmd/go/internal/imports/testdata/illumos/f.go

+// +build solaris
+
+package illumos
+
+import _ "f"

src/cmd/go/internal/imports/testdata/illumos/g.go

+// +build !illumos
+
+package illumos
+
+import _ "g"

src/cmd/go/internal/imports/testdata/illumos/tags.txt

+illumos amd64

src/cmd/go/internal/imports/testdata/illumos/want.txt

+a
+b
+c
+d
+e
+f