cmd/go: parse dot-separated identifiers in build metadata

Per https://semver.org/#spec-item-10, build metadata may include
a series of dot separated identifiers.

Fixes #28647

Change-Id: I98655c62584a822953df71fba32b4a2cafe7a04b
Reviewed-on: https://go-review.googlesource.com/c/148835Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/semver/semver.go

@@ -263,7 +263,7 @@ func parseBuild(v string) (t, rest string, ok bool) {
 	i := 1
 	start := 1
 	for i < len(v) {
-		if !isIdentChar(v[i]) {
+		if !isIdentChar(v[i]) && v[i] != '.' {
 			return
 		}
 		if v[i] == '.' {

src/cmd/go/internal/semver/semver_test.go

@@ -44,6 +44,7 @@ var tests = []struct {
 	{"v1.2.3", "v1.2.3"},
 	{"v1.2.3+meta", "v1.2.3"},
 	{"v1.2.3+meta-pre", "v1.2.3"},
+	{"v1.2.3+meta-pre.sha.256a", "v1.2.3"},
 }
 
 func TestIsValid(t *testing.T) {