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/androidmk/Android.bp b/androidmk/Android.bp
index 79fe530..4110073 100644
--- a/androidmk/Android.bp
+++ b/androidmk/Android.bp
@@ -19,12 +19,23 @@
 blueprint_go_binary {
     name: "androidmk",
     srcs: [
-        "cmd/androidmk/android.go",
-        "cmd/androidmk/androidmk.go",
-        "cmd/androidmk/values.go",
+        "cmd/androidmk.go",
+    ],
+    deps: [
+        "androidmk-lib",
+    ],
+}
+
+bootstrap_go_package {
+    name: "androidmk-lib",
+    pkgPath: "android/soong/androidmk/androidmk",
+    srcs: [
+        "androidmk/android.go",
+        "androidmk/androidmk.go",
+        "androidmk/values.go",
     ],
     testSrcs: [
-        "cmd/androidmk/androidmk_test.go",
+        "androidmk/androidmk_test.go",
     ],
     deps: [
         "androidmk-parser",