entc: abandon plugins

Summary:
Go plugin is not a good solution for both internal and external usage.
It's hard to manage and maintain matching versions (both Go and external libraries), and it does not support Windows.

Reviewed By: alexsn

Differential Revision: D16582217

fbshipit-source-id: 81876d2c6f30bbfc16ecf9e5000f0670f2e62484

entc/load/testdata/invalid/schema.go

@@ -0,0 +1,4 @@
+package invalid
+
+// User is invalid schema.
+type User struct{}