Make duplicate-related warning fatal in developer mode

Fixes #605

I've thought of another variant using macro - it has less copypasta, but hides a return which is not really good. But if you find it more applicable, I can modify the request.

Macro:
```
#define DUPLICATE_WARNING(format, ...) do { \
  bool developer; \
  pkg_config_bool(PKG_CONFIG_DEVELOPER_MODE, &developer); \
  if (developer) { \
    pkg_emit_error(format", fatal (developer mode)", __VA_ARGS__); \
    return (EPKG_FATAL); \
  } else { \
    pkg_emit_error(format", ignoring", __VA_ARGS__); \
  } \
} while(0)
```
Usage:
```
  if (l != NULL) {
    DUPLICATE_WARNING("duplicate license listing: %s", name);
    return (EPKG_OK);
  }
```