Make PATH scanning/filtering a native operation

[stuhood]

(broken out of #9760)

The ability to adjust the PATH (filtering) for a process is a fairly universal thing, and a global default for processes is likely. But it will likely also be necessary to adjust this on a Process by Process basis (a simpler form of what was described in #10156).

So, we should:

1. Add a ProcessEnvironment subsystem with an option to override/filter the PATH.

   • And probably rename the SubprocessEnvironment subsystem (which is somewhat Python specific) to minimize confusion.
   • And lift the PATH off of PEXEnvironment onto the global ProcessEnvironment
   • Both of these will eventually become platform specific via Platform/Environment-specific subsystems #7735.

2. Add Process(.., environment: Optional[ProcessEnvironment]), and adjust the Process intrinsic to depend on the global ProcessEnvironment if an environment is not specified.

   • Each rust CommandRunner will do "Something" to detect the PATH in the relevant environment, and will then filter it to provide to all processes:
     • For the local CommandRunner, this will mean adding a complete PATH global_option (marked daemon=True to restart pantsd, and with some deeper scanning), with a default of the entire PATH that pants is running with. That variable will exist for fingerprinting purposes, and is the thing that will then be filtered down on a process by process basis by the PATH filter option.
     • For the remote CommandRunner, this will involve running a specially formulated process lazily at startup time to detect the PATH in the remote environment, and then filtering that for each user process that runs afterward.

3. Implement a syntax for declaring a scoped Subsystem dependency, so that different processes can choose to use different instances of the subsystem.

   • Most likely via memoization of an anonymous class:

   MyProcessEnvironment = ProcessEnvironment.scoped_to("my_scope")
   
   SubsystemRule(MyProcessEnvironment)
   

   • This is OK as a followup: if people can configure global PATH filters, we can migrate toward having scoped configuration later (similar to Platform/Environment-specific subsystems #7735 later introducing environment-specific filters).

---

#10489 did a portion of this by adding the PEXEnvironment Subsystem with a PATH-equivalent variable. But it has a caching bug: a particular PATH will be filtered exactly once, even though its content might change for each run.

[stuhood]

cc @tdyas : This relates to the discussion of remote caching yesterday.