Bazel: Move include paths to libraries

Instead of having a global list of -I include paths, have each library
provide a -I for the public/ directory using the includes argument.

Change-Id: I2e321752906920cd94beaaab97b8796af85078e4
diff --git a/pw_varint/BUILD b/pw_varint/BUILD
index c1b0522..abfa158 100644
--- a/pw_varint/BUILD
+++ b/pw_varint/BUILD
@@ -30,6 +30,7 @@
     hdrs = [
         "public/pw_varint/varint.h",
     ],
+    includes = ["public"],
     deps = [
         "//pw_span",
     ],