Loading a new core.Process in core.Core is a rather long, complex
process. Currently, this function creates a stub Process with very few
fields set, and then subsequent method calls initially the remaining
fields.

The problem with this approach is that the inputs/requirements and
outputs of these initialization methods is poorly documented. These
methods (e.g., readCore) usually have some prerequisite fields in Process
that must be set on entry, and some fields that they set prior to
return.

Neither of these are well documented, and many are far from obvious. For
example, Process.mainExecName is initialized in readCore -> readNote ->
readNTFile -> openMappedFile.

This is made more of a mess by the fact that Process tries to do most
initialization with a single pass, resulting in fields getting
initialized in unintuitive locations (such as mainExecName).

These combine to make refactoring to support new functionality
difficult, as changing ordering breaks implicit dependencies between
methods.

This CL addresses these issues by eliminating the iterative
initialization of Process. Instead, Process is only instantiated once
all fields are ready. During loading, all work is done by free
functions/types, where inputs and outputs are explicit via function
arguments/returns.

This may inadvertently improve golang/go#44757, as it makes main binary
lookup more explicit.

This CL intends to keep behavior the same, but there are a few changes:

* If the entry point is unknown, We no longer apply the heuristic of
  assuming that first executable mapping is the main binary. This could
  be added back, but it is probably better to ensure that the entry
  point is available on all architectures.

* Failure to ELF parse a file for symbols isn't a hard error (as the
  mapped file might not even be an ELF).

For golang/go#57447.

Change-Id: I7c3712ccb99ceddddc6adbae57d477c25ff4e5ba
Reviewed-on: https://go-review.googlesource.com/c/debug/+/506558
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>