Start unblocking com.android.runtime deps

Adds support to warn rather than error about missing deps in order to
suport modules in build/soong with deps outside of build/sooong in
bp2build integration tests.

Test: build/bazel/ci/bp2build.sh
Change-Id: I1282bccd37a3fc9f33555d34e68d7f0873d8272c

bp2build/build_conversion_test.go

@@ -300,6 +300,19 @@ custom {
 				}),
 			},
 		},
+		{
+			description: "non-existent dep",
+			blueprint: `custom {
+  name: "has_dep",
+  arch_paths: [":dep"],
+  bazel_module: { bp2build_available: true },
+}`,
+			expectedBazelTargets: []string{
+				makeBazelTarget("custom", "has_dep", attrNameToString{
+					"arch_paths": `[":dep__BP2BUILD__MISSING__DEP"]`,
+				}),
+			},
+		},
 		{
 			description: "arch-variant srcs",
 			blueprint: `custom {