refactor: move more commands to internal/cmd (#207)

* refactor: move more commands to internal/cmd

Part of https://github.com/ooni/probe/issues/1335.

We would like all commands to be at the same level of engine
rather than inside engine (now that we can do it).

* fix: update .gitignore

* refactor: also move jafar outside engine

* We should be good now?

internal/engine/httpheader/accept.go

@@ -0,0 +1,6 @@
+package httpheader
+
+// Accept returns the Accept header used for measuring.
+func Accept() string {
+	return "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
+}

internal/engine/httpheader/acceptlanguage.go

@@ -0,0 +1,6 @@
+package httpheader
+
+// AcceptLanguage returns the Accept-Language header used for measuring.
+func AcceptLanguage() string {
+	return "en-US;q=0.8,en;q=0.5"
+}

internal/engine/httpheader/useragent.go

@@ -0,0 +1,16 @@
+// Package httpheader contains code to set common HTTP headers.
+package httpheader
+
+// UserAgent returns the User-Agent header used for measuring.
+func UserAgent() string {
+	// 12.0% as of Jan 29, 2021 according to https://techblog.willshouse.com/2012/01/03/most-common-user-agents/
+	const ua = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.141 Safari/537.36"
+	return ua
+}
+
+// CLIUserAgent returns the User-Agent used when we want to
+// pretent to be a command line HTTP client.
+func CLIUserAgent() string {
+	// here we always put the latest version of cURL.
+	return "curl/7.73.0"
+}