Backport Bitcoin PR#7348: MOVE ONLY: move rpc* to rpc/ + same for Dash-specific rpc

[OlegGirko]

This is backport of Bitcoin PR bitcoin#7348.

Original description follows.

Was just playing around with the RPC, and found the constant prefixing of file names frustrating.

This also seems to be in line with the way other modules (zmq etc) have been grouped.

Should be easy to review.

Note: This won't negatively impact git blame

[tgflynn]

utACK

Logically there should be a separate PR to move the dash specific rpc code to rpc/.

[UdjinM6]

PR title and description are misleading since this also moves Dash-specific rpc files and probably should be fixed. Otherwise looks good.

[OlegGirko]

For me backporting means not just blindly applying changes, but also making all commits look like they were applied to Dash codebase originally. This means adapting Dash-specific parts as well. Without that I would call it "partial backport".

In this case there is a commit that renames all rpc*.{h,cpp} files to rpc/*.{h,cpp}. It would be illogical to rename only files mentioned in original commit and forget to rename a few Dash-specific files.

Of course, I'll try to be more clear how commit was changed to affect Dash-specific code in further backports. But does it make sense to always add "and adapt Dash-specific parts" to backport PR descriptions?