Major plugin refactor and cleanup.

Switched to POCO library for unified platform/library interface.
Deprecated the external module API. It was creating more problems than solving.
Removed most built-in libraries in favor of system libraries for easier maintenance.
Cleaned and secured code with help from static analyzers.

vendor/CPR/cpr/error.cpp

@@ -0,0 +1,68 @@
+#include "cpr/error.h"
+
+#include <curl/curl.h>
+
+namespace cpr {
+ErrorCode Error::getErrorCodeForCurlError(std::int32_t curl_code) {
+    switch (curl_code) {
+        case CURLE_OK:
+            return ErrorCode::OK;
+        case CURLE_UNSUPPORTED_PROTOCOL:
+            return ErrorCode::UNSUPPORTED_PROTOCOL;
+        case CURLE_URL_MALFORMAT:
+            return ErrorCode::INVALID_URL_FORMAT;
+        case CURLE_COULDNT_RESOLVE_PROXY:
+            return ErrorCode::PROXY_RESOLUTION_FAILURE;
+        case CURLE_COULDNT_RESOLVE_HOST:
+            return ErrorCode::HOST_RESOLUTION_FAILURE;
+        case CURLE_COULDNT_CONNECT:
+            return ErrorCode::CONNECTION_FAILURE;
+        case CURLE_OPERATION_TIMEDOUT:
+            return ErrorCode::OPERATION_TIMEDOUT;
+        case CURLE_SSL_CONNECT_ERROR:
+            return ErrorCode::SSL_CONNECT_ERROR;
+#if LIBCURL_VERSION_NUM < 0x073e00
+        case CURLE_PEER_FAILED_VERIFICATION:
+            return ErrorCode::SSL_REMOTE_CERTIFICATE_ERROR;
+#endif
+        case CURLE_ABORTED_BY_CALLBACK:
+        case CURLE_WRITE_ERROR:
+            return ErrorCode::REQUEST_CANCELLED;
+        case CURLE_GOT_NOTHING:
+            return ErrorCode::EMPTY_RESPONSE;
+        case CURLE_SSL_ENGINE_NOTFOUND:
+        case CURLE_SSL_ENGINE_SETFAILED:
+            return ErrorCode::GENERIC_SSL_ERROR;
+        case CURLE_SEND_ERROR:
+            return ErrorCode::NETWORK_SEND_FAILURE;
+        case CURLE_RECV_ERROR:
+            return ErrorCode::NETWORK_RECEIVE_ERROR;
+        case CURLE_SSL_CERTPROBLEM:
+            return ErrorCode::SSL_LOCAL_CERTIFICATE_ERROR;
+        case CURLE_SSL_CIPHER:
+            return ErrorCode::GENERIC_SSL_ERROR;
+#if LIBCURL_VERSION_NUM >= 0x073e00
+        case CURLE_PEER_FAILED_VERIFICATION:
+            return ErrorCode::SSL_REMOTE_CERTIFICATE_ERROR;
+#else
+        case CURLE_SSL_CACERT:
+            return ErrorCode::SSL_CACERT_ERROR;
+#endif
+        case CURLE_USE_SSL_FAILED:
+        case CURLE_SSL_ENGINE_INITFAILED:
+            return ErrorCode::GENERIC_SSL_ERROR;
+        case CURLE_SSL_CACERT_BADFILE:
+            return ErrorCode::SSL_CACERT_ERROR;
+        case CURLE_SSL_SHUTDOWN_FAILED:
+            return ErrorCode::GENERIC_SSL_ERROR;
+        case CURLE_SSL_CRL_BADFILE:
+        case CURLE_SSL_ISSUER_ERROR:
+            return ErrorCode::SSL_CACERT_ERROR;
+        case CURLE_TOO_MANY_REDIRECTS:
+            return ErrorCode::OK;
+        default:
+            return ErrorCode::INTERNAL_ERROR;
+    }
+}
+
+} // namespace cpr