[mlir] support external named transform libraries

Introduce support for external definitions of named sequences in the
transform dialect by letting the TransformInterpreterPassBase read a
"library" MLIR file. This file is expected to contain definitions for
named sequences that are only declared in the main transformation
script. This allows for sharing non-trivial transform combinations
without duplication.

This patch provides only the minimal plumbing for a single textual IR
file. Further changes are possible to support multiple libraries and
bytecode files.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D146961