Add support to extend commands bpfix and androidmk

Allows our partners to maintain partner-specific fixes in their repos. Converts most of androidmk into a library similar to bpfix. Makes some methods and types public for use by extended versions.

Bug:140727544
Test: Ran unit test cases &&
  ran test conversions with sample
Change-Id: I7e1fbf3a6a7a8bd47334f43fe3eb68cbbd3426c1
diff --git a/bpfix/Android.bp b/bpfix/Android.bp
index 90a453d..aec9ff9 100644
--- a/bpfix/Android.bp
+++ b/bpfix/Android.bp
@@ -19,7 +19,18 @@
 blueprint_go_binary {
     name: "bpfix",
     srcs: [
-        "cmd/bpfix.go",
+        "cmd/main.go",
+    ],
+    deps: [
+        "bpfix-cmd",
+    ],
+}
+
+bootstrap_go_package {
+    name: "bpfix-cmd",
+    pkgPath: "android/soong/bpfix/bpfix/cmd",
+    srcs: [
+        "cmd-lib/bpfix.go",
     ],
     deps: [
         "bpfix-lib",