Add more plugin integration tests

Motivation:

The SwiftPM build plugin doesn't currently handle proto files with the
same name even if they come from different packages. This happens
because the build system cannot handle object files with the same name.
This change adds such a test while tidying up some of the plugin test
code.

Modifications:

- Divide up test resources in sources, config, and proto directories.
- Have one function per test
- Document each test with the expected structure
- Add another test but don't run it. (It fails.)

Result:

More tests. The issue will be fixed in a separate change.

Author: glbrntt

IntegrationTests/PluginTests/Resources/import-directory-1-grpc-swift-proto-generator-config.json renamed to IntegrationTests/PluginTests/Resources/Config/import-directory-1-grpc-swift-proto-generator-config.json

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+import "google/protobuf/empty.proto";
+
+package noop;
+
+service NoOpService {
+  rpc NoOp(google.protobuf.Empty) returns (google.protobuf.Empty);
+}

IntegrationTests/PluginTests/Resources/internal-grpc-swift-proto-generator-config.json renamed to IntegrationTests/PluginTests/Resources/Config/internal-grpc-swift-proto-generator-config.json

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+import "google/protobuf/empty.proto";
+
+package noop2;
+
+service NoOpService {
+  rpc NoOp(google.protobuf.Empty) returns (google.protobuf.Empty);
+}