[mlir][transform] Support symlinks in module loading. Reorganize tests. (#69329)

A recent commit (#69190) broke the bazel builds. Turns out that Bazel
uses symlinks for providing the test files, which the path expansion of
the module loading mechanism did not handle correctly. This PR fixes
that.

It also reorganizes the tests better: It puts all `.mlir` files that are
included by some other test into a common `include` folder. This greatly
simplifies the definition of the dependencies between the different
`.mlir` files in Bazel's `BUILD` file. The commit also adds a comment to
all included files why these aren't tested themselves direclty and uses
the `%{fs-sep}` expansion for paths more consistently. Finally, it
uncomments all but one of the tests excluded in Bazel because they seem
to run now. (The remaining one includes a file that it itself a test, so
it would have to live *in* and *outside* of the `include` folder.)