rpc: Allow single transaction through submitpackage

And under the hood suppoert single transactions
in AcceptPackage. This simplifies user experience
and paves the way for reducing number of codepaths
for transaction acceptance in the future.

Co-Authored-By: instagibbs <gsanders87@gmail.com>

doc/policy/packages.md

@@ -76,7 +76,7 @@ The following rules are only enforced for packages to be submitted to the mempoo
 enforced for test accepts):
 
 * Packages must be child-with-unconfirmed-parents packages. This also means packages must contain at
-  least 2 transactions. (#22674)
+  least 1 transaction. (#31096)
 
    - *Rationale*: This allows for fee-bumping by CPFP. Allowing multiple parents makes it possible
      to fee-bump a batch of transactions. Restricting packages to a defined topology is easier to