cmd/gomobile: reorganize build logic

The goal here is to remove several inconsistencies between
-target=android and -target=ios support, along with making the flow
of the command follow the path you might expect given a certain set
of flags, and preparing for `gomobile bind` support of ios. In
particular, building non-main packages now works with both targets
and the initialization of global build state is clearer.

The reorg also is designed around an nm trick I thought of
yesterday to do better package import scanning without a slow
all-file scan. This will give better detection of x/mobile/app and
x/mobile/exp/audio/al packages. There's a TODO about it, and I'll do
it in a future CL.

Tested with:

	go test golang.org/x/mobile/cmd/gomobile
	gomobile init
	gomobile bind golang.org/x/mobile/asset
	go test golang.org/x/mobile/bind/java
	gomobile build -target=ios golang.org/x/mobile/example/basic
	gomobile build -target=ios golang.org/x/mobile/gl
	gomobile build -target=android golang.org/x/mobile/gl
	gomobile build -target=android golang.org/x/mobile/example/basic
	(Along with manual testing of basic on an android device.)

That might make a pretty good _test.go.

Change-Id: I41230008c3c15db25a11c33b9eaca4abada9f411
Reviewed-on: https://go-review.googlesource.com/12051
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/Seq.java

@@ -21,6 +21,7 @@ public class Seq {
 			Class.forName("go.LoadJNI");
 		} catch (ClassNotFoundException e) {
 			// Ignore, assume the user will load JNI for it.
+			Log.w("GoSeq", "LoadJNI class not found");
 		}
 
 		initSeq();
@@ -239,7 +240,7 @@ public class Seq {
 			if (refnum <= 0) {
 				// We don't keep track of the Go object.
 				// This must not happen.
-				Log.wtf("Seq", "dec request for Go object "+ refnum);
+				Log.wtf("GoSeq", "dec request for Go object "+ refnum);
 				return;
 			}
 			// Java objects are removed on request of Go.