go: include vendored modules

RHEL requires the source code for dependencies to be included in the
srpm. The spec file already expects that, but we've only included the
vendored modules (i.e., the `vendor` directory) in the `rhel-8.2.`
branch. Move vendoring to master, so that we can build RHEL packages
from it as well.

This commit is the result of running `go mod vendor`, which includes the
vendored sources and updates go.mod and go.sum files.

Fedora requires the opposite: dependencies should not be vendored. The
spec file already ignores the `vendor` directory by default.

vendor/github.com/mattn/go-ieproxy/proxyMiddleman_windows.go

@@ -0,0 +1,51 @@
+package ieproxy
+
+import (
+	"net/http"
+	"net/url"
+
+	"golang.org/x/net/http/httpproxy"
+)
+
+func proxyMiddleman() func(req *http.Request) (i *url.URL, e error) {
+	// Get the proxy configuration
+	conf := GetConf()
+	envcfg := httpproxy.FromEnvironment()
+
+	if envcfg.HTTPProxy != "" || envcfg.HTTPSProxy != "" {
+		// If the user manually specifies environment variables, prefer those over the Windows config.
+		return http.ProxyFromEnvironment
+	} else if conf.Automatic.Active {
+		// If automatic proxy obtaining is specified
+		return func(req *http.Request) (i *url.URL, e error) {
+			host := conf.Automatic.FindProxyForURL(req.URL.String())
+			if host == "" {
+				return nil, nil
+			}
+			return &url.URL{Host: host}, nil
+		}
+	} else if conf.Static.Active {
+		// If static proxy obtaining is specified
+		prox := httpproxy.Config{
+			HTTPSProxy: mapFallback("https", "", conf.Static.Protocols),
+			HTTPProxy:  mapFallback("http", "", conf.Static.Protocols),
+			NoProxy:    conf.Static.NoProxy,
+		}
+
+		return func(req *http.Request) (i *url.URL, e error) {
+			return prox.ProxyFunc()(req.URL)
+		}
+	} else {
+		// Final fallthrough case; use the environment variables.
+		return http.ProxyFromEnvironment
+	}
+}
+
+// Return oKey or fbKey if oKey doesn't exist in the map.
+func mapFallback(oKey, fbKey string, m map[string]string) string {
+	if v, ok := m[oKey]; ok {
+		return v
+	} else {
+		return m[fbKey]
+	}
+}