Re-organize project structure to separate PyTorch dependencies from core project.

[stellaraccident]

This is a first step towards the structure we discussed here: https://gist.github.com/stellaraccident/931b068aaf7fa56f34069426740ebf20

There are two primary goals:

1. Separate the core project (C++ dialects and conversions) from the hard PyTorch dependencies. We move all such things into projects/pt1 as a starting point since they are presently entangled with PT1-era APIs. Additional work can be done to disentangle components from that (specifically LTC is identified as likely ultimately living in a projects/ltc).
2. Create space for native PyTorch2 Dynamo-based infra to be upstreamed without needing to co-exist with the original TorchScript path.

Very little changes in this path with respect to build layering or options. These can be updated in a followup without commingling directory structure changes.

This also takes steps toward a couple of other layering enhancements:

• Removes the llvm-external-projects/torch-mlir-dialects sub-project, collapsing it into the main tree.
• Audits and fixes up the core C++ build to account for issues found while moving things. This is just an opportunistic pass through but roughly ~halves the number of build actions for the project from the high 4000's to the low 2000's.

It deviates from the discussed plan by having a projects/ tree instead of compat/. As I was thinking about it, this will better accommodate the follow-on code movement.

Once things are roughly in place and the CI passing, followups will focus on more in-situ fixes and cleanups.