Add imports property to py_library rules

This is to avoid having it hardcoded in a fork of the
py_library rule.

Most import attributes should just be set to ".", but
our previous solution always hardcoded it to ".." instead,
for ndkstubgen. ndkstubgen uses pkg_path: "ndkstubgen",
i.e., it set pkg_path to the name of the folder that
contained the Android.bp file. In this specific scenario,
imports = ".." works. Recreate that behavior here as well,
because we don't handle pkg_path properly yet.

Fixes: 233081071
Test: build/bazel/ci/bp2build.sh
Change-Id: Ib5e6a8edf428c74d4b5947f0ec53a2151001367a
diff --git a/bp2build/testing.go b/bp2build/testing.go
index 029ba49..9341495 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -119,8 +119,8 @@
 		return
 	}
 
-	errs := append(parseErrs, resolveDepsErrs...)
-	if tc.expectedErr != nil && checkError(t, errs, tc.expectedErr) {
+	parseAndResolveErrs := append(parseErrs, resolveDepsErrs...)
+	if tc.expectedErr != nil && checkError(t, parseAndResolveErrs, tc.expectedErr) {
 		return
 	}
 
@@ -135,7 +135,7 @@
 		if checkError(t, errs, tc.expectedErr) {
 			return
 		} else {
-			t.Errorf("Expected error: %q, got: %q", tc.expectedErr, errs)
+			t.Errorf("Expected error: %q, got: %q and %q", tc.expectedErr, errs, parseAndResolveErrs)
 		}
 	} else {
 		android.FailIfErrored(t, errs)