Rewriting/simplifying DynamicLibrary in C.

This allowed for a lot of file IO code to go away - there was needless
abstraction here as there was only a single user of a lot of these things
that was already platform-specialized.

Progress on #4369 and #3848.
Fixes #4642.
Unblocks #3845, which can now be added cleanly.

# Conflicts:
#	iree/hal/local/loaders/legacy_library_loader.c

iree/base/BUILD

@@ -63,26 +63,6 @@ cc_library(
 # Internal IREE C++ wrappers and utilities
 #===------------------------------------------------------------------------===#
 
-cc_library(
-    name = "dynamic_library",
-    srcs = [
-        "dynamic_library_posix.cc",
-        "dynamic_library_win32.cc",
-    ],
-    hdrs = ["dynamic_library.h"],
-    deps = [
-        ":core_headers",
-        ":logging",
-        ":status",
-        ":tracing",
-        "//build_tools:default_linkopts",
-        "//iree/base/internal:file_path",
-        "@com_google_absl//absl/memory",
-        "@com_google_absl//absl/strings",
-        "@com_google_absl//absl/types:span",
-    ],
-)
-
 cc_library(
     name = "flatcc",
     hdrs = ["flatcc.h"],

iree/base/CMakeLists.txt

@@ -48,26 +48,6 @@ iree_cc_library(
   PUBLIC
 )
 
-iree_cc_library(
-  NAME
-    dynamic_library
-  HDRS
-    "dynamic_library.h"
-  SRCS
-    "dynamic_library_posix.cc"
-    "dynamic_library_win32.cc"
-  DEPS
-    ::core_headers
-    ::logging
-    ::status
-    ::tracing
-    absl::memory
-    absl::span
-    absl::strings
-    iree::base::internal::file_path
-  PUBLIC
-)
-
 iree_cc_library(
   NAME
     flatcc