Adapt the _ALT style to our new standard

- in .h files: only put the context declaration inside the #ifdef _ALT
  (this was changed in 2.9.0, ie after the original PR)
- in .c file: only leave selftest out of _ALT: even though some function are
  trivial to build from other parts, alt implementors might want to go another
way about them (for efficiency or other reasons)
diff --git a/include/mbedtls/aead_chacha20_poly1305.h b/include/mbedtls/aead_chacha20_poly1305.h
index 6c8e420..6f7ab6f 100644
--- a/include/mbedtls/aead_chacha20_poly1305.h
+++ b/include/mbedtls/aead_chacha20_poly1305.h
@@ -29,11 +29,6 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
-#if !defined(MBEDTLS_AEAD_CHACHA20_POLY1305_ALT)
-
-#include "chacha20.h"
-#include "poly1305.h"
-
 #define MBEDTLS_ERR_AEAD_CHACHA20_POLY1305_BAD_INPUT_DATA -0x00047 /**< Invalid input parameter(s). */
 #define MBEDTLS_ERR_AEAD_CHACHA20_POLY1305_BAD_STATE      -0x00049 /**< The requested operation is not permitted in the current state */
 
@@ -44,6 +39,11 @@
 }
 mbedtls_aead_chacha20_poly1305_mode_t;
 
+#if !defined(MBEDTLS_AEAD_CHACHA20_POLY1305_ALT)
+
+#include "chacha20.h"
+#include "poly1305.h"
+
 typedef struct
 {
     mbedtls_chacha20_context chacha20_ctx;      /** ChaCha20 context */
@@ -55,6 +55,10 @@
 }
 mbedtls_aead_chacha20_poly1305_context;
 
+#else /* !MBEDTLS_AEAD_CHACHA20_POLY1305_ALT */
+#include "aead_chacha20_poly1305_alt.h"
+#endif /* !MBEDTLS_AEAD_CHACHA20_POLY1305_ALT */
+
 /**
  * \brief               Initialize ChaCha20-Poly1305 context
  *
@@ -183,10 +187,6 @@
 int mbedtls_aead_chacha20_poly1305_finish( mbedtls_aead_chacha20_poly1305_context *ctx,
                                            unsigned char mac[16] );
 
-#else /* !MBEDTLS_AEAD_CHACHA20_POLY1305_ALT */
-#include "aead_chacha20_poly1305_alt.h"
-#endif /* !MBEDTLS_AEAD_CHACHA20_POLY1305_ALT */
-
 /**
  * \brief               Encrypt or decrypt data, and produce a MAC with ChaCha20-Poly1305.
  *