Clean up the P4-14 dependent code.

[fruffy]

First attempt to make P4-14 specific code optional in the compiler. This can make it a little easier to reason about IR changes and might reduce compile times, too.

The goal is to remove all "v1model"-specific code from the compiler core and move it into the appropriate folders. What makes this change a little tedious is that https://github.com/p4lang/p4c/blob/main/frontends/p4/fromv1.0/v1model.h is a bunch of constant strings that are referenced by the control plane and the toP4 pass.

In subsequent PRs, I will try to find a replacement for this file.

[fruffy]

With #4899 we can likely get rid of this code and move it into the bmv2 back end.

[grg]

We also have a P4-16 implementation of the v1model (p4include/v1model.p4). I'm not sure how many of the declarations you've moved are used by v1model.p4, but is the expectation that these won't compile if the P4-14 code is excluded?

[fruffy]

We also have a P4-16 implementation of the v1model (p4include/v1model.p4). I'm not sure how many of the declarations you've moved are used by v1model.p4, but is the expectation that these won't compile if the P4-14 code is excluded?

The code doesn't seem to be used much in the P4-16 parts except for the constants in v1model.h but I will know whether better there are IR references once I actually disable it. If that is the case, these IR nodes should be part of the bmv2 def files. Otherwise, the changes here are fairly safe.

But I do not know how Tofino depends on the code here.