Gazelle select (7/x): platform constraints (#494)

Add a table of platforms Gazelle should generate BUILD files for. Each
platform has a label of a predefined config_setting rule and a list of
tags that are true on that platform. 

At this time, platforms are not configurable, but they may be in the
future if we find a compelling use case.

Related #409

go/platform/BUILD

diff --git a/go/platform/BUILD b/go/platform/BUILD
new file mode 100644
index 0000000..9eeee04
--- /dev/null
+++ b/go/platform/BUILD
@@ -0,0 +1,29 @@
+package(default_visibility = ["//visibility:public"])
+
+# This file declares a config_setting for each supported platform.
+# These can be used in calls to select to choose platform-specific sources
+# and dependencies.
+#
+# Eventually, we hope to be able to use platform or constraint_value instead,
+# but Bazel doesn't support this yet.
+
+config_setting(
+    name = "darwin_amd64",
+    values = {
+        "cpu": "darwin",
+    },
+)
+
+config_setting(
+    name = "linux_amd64",
+    values = {
+        "cpu": "k8",
+    },
+)
+
+config_setting(
+    name = "windows_amd64",
+    values = {
+        "cpu": "x64_windows_msvc",
+    },
+)

go/tools/gazelle/generator/generator.go

diff --git a/go/tools/gazelle/generator/generator.go b/go/tools/gazelle/generator/generator.go
index a2df4ca..cd9a4e4 100644
--- a/go/tools/gazelle/generator/generator.go
+++ b/go/tools/gazelle/generator/generator.go
@@ -40,6 +40,7 @@
 	goPrefix      string
 	buildFileName string
 	bctx          build.Context
+	platforms     packages.PlatformConstraints
 	g             rules.Generator
 }
 
@@ -77,6 +78,7 @@
 		goPrefix:      goPrefix,
 		buildFileName: buildFileName,
 		bctx:          bctx,
+		platforms:     packages.DefaultPlatformConstraints,
 		g:             rules.NewGenerator(repoRoot, goPrefix, external),
 	}, nil
 }

go/tools/gazelle/packages/package.go

diff --git a/go/tools/gazelle/packages/package.go b/go/tools/gazelle/packages/package.go
index 0e77744..f55103b 100644
--- a/go/tools/gazelle/packages/package.go
+++ b/go/tools/gazelle/packages/package.go
@@ -15,7 +15,29 @@
 
 package packages
 
-import "strings"
+import (
+	"fmt"
+	"strings"
+)
+
+// PlatformConstraints is a map from config_setting labels (for example,
+// "@io_bazel_rules_go//go/platform:linux_amd64") to a sets of build tags
+// that are true on each platform (for example, "linux,amd64").
+type PlatformConstraints map[string]map[string]bool
+
+// DefaultPlatformConstraints is the default set of platforms that Gazelle
+// will generate files for. These are the platforms that both Go and Bazel
+// support.
+var DefaultPlatformConstraints PlatformConstraints
+
+func init() {
+	DefaultPlatformConstraints = make(PlatformConstraints)
+	arch := "amd64"
+	for _, os := range []string{"darwin", "linux", "windows"} {
+		label := fmt.Sprintf("@io_bazel_rules_go//go/platform:%s_%s", os, arch)
+		DefaultPlatformConstraints[label] = map[string]bool{arch: true, os: true}
+	}
+}
 
 // Package contains metadata about a Go package extracted from a directory.
 // It fills a similar role to go/build.Package, but it separates files by