[mlir][Linalg] NFC - Cleanup Linalg Pass locations and namespacing

[mlir][Linalg] NFC - Cleanup Linalg Pass locations and namespacing

Summary:
This diff moves the conversion pass declaration closer to its definition
and makes the namespacing of passes consistent with the rest of the
infrastructure (i.e. `mlir::linalg::createXXXPass` -> `mlir::createXXXPass`).

Reviewers: ftynse, jpienaar, mehdi_amini

Subscribers: rriddle, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72766