Remove all (non-dev) client references from frame, activate dependency enforcer (#4184)

* Move transaction pool to primitives

* move backend, errors into primitives

* remove unused client depencies

* Move rpc-api into primitives

* Move peerset back to client

* Move rpc/api back to client, move palette/support/rpc into utils

* move support-rpc into subfolder

* move system-rpc into utils

* move transaction-pool  and -graph back into client

* fix broken imports

* Clean up test primitives

* Make support test utils independent of frame

* remove unnecessary node dependencies from service

* Reactivate dependency script:
 - only enforce the now achieved status quo will remain
 - allow for primitives to depend on /client for now without failing
 - more discriptive error message so people understand, what it wants
 - minor fix to differentiative between ../client and /client (which may be a subfolder)
 - don't allow this to fail anylonger.

* fix doc comment

* 'Should not' rather than 'must not'.

* Revert unwanted dependency changes

* fix faulty import

* fixup derive_more version

* fix wrong import path

substrate/frame/elections-phragmen/Cargo.toml

@@ -17,6 +17,7 @@ runtime_io = { package = "sr-io", path = "../../primitives/sr-io" }
 hex-literal = "0.2.1"
 balances = { package = "pallet-balances", path = "../balances" }
 primitives = { package = "substrate-primitives",  path = "../../primitives/core" }
+substrate-test-utils = { path = "../../test/utils" }
 serde = { version = "1.0.101" }
 
 [features]