Stop splitting sources in emit_archive (#4015)

We split them up only to combine all 7 lists again. It doesn't seem
useful.

**What type of PR is this?**
Perf improvement/cleanup

**What does this PR do? Why is it needed?**

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

Author: dzbarsky

go/private/actions/archive.bzl

@@ -15,7 +15,6 @@
 load(
     "//go/private:common.bzl",
     "as_tuple",
-    "split_srcs",
 )
 load(
     "//go/private:mode.bzl",
@@ -45,7 +44,6 @@ def emit_archive(go, source = None, _recompile_suffix = "", recompile_internal_d
     if source == None:
         fail("source is a required parameter")
 
-    split = split_srcs(source.srcs)
     testfilter = getattr(source.library, "testfilter", None)
     pre_ext = ""
     if go.mode.link == LINKMODE_C_ARCHIVE:
@@ -92,7 +90,7 @@ def emit_archive(go, source = None, _recompile_suffix = "", recompile_internal_d
         clinkopts = [f for fs in source.clinkopts for f in fs.split(" ")]
         cgo = cgo_configure(
             go,
-            srcs = split.go + split.c + split.asm + split.cxx + split.objc + split.headers + split.syso,
+            srcs = source.srcs,
             cdeps = source.cdeps,
             cppopts = cppopts,
             copts = copts,
@@ -105,7 +103,7 @@ def emit_archive(go, source = None, _recompile_suffix = "", recompile_internal_d
         runfiles = runfiles.merge(cgo.runfiles)
         emit_compilepkg(
             go,
-            sources = split.go + split.c + split.asm + split.cxx + split.objc + split.headers + split.syso,
+            sources = source.srcs,
             cover = source.cover,
             embedsrcs = source.embedsrcs,
             importpath = importpath,
@@ -134,7 +132,7 @@ def emit_archive(go, source = None, _recompile_suffix = "", recompile_internal_d
         cgo_deps = depset()
         emit_compilepkg(
             go,
-            sources = split.go + split.c + split.asm + split.cxx + split.objc + split.headers + split.syso,
+            sources = source.srcs,
             cover = source.cover,
             embedsrcs = source.embedsrcs,
             importpath = importpath,

go/private/common.bzl

@@ -110,10 +110,6 @@ def split_srcs(srcs):
         extouts.append(src)
     return sources
 
-def join_srcs(source):
-    """Combines source from a split_srcs struct into a single list."""
-    return source.go + source.headers + source.asm + source.c + source.cxx + source.objc + source.syso
-
 def os_path(ctx, path):
     path = str(path)  # maybe convert from path type
     if ctx.os.name.startswith("windows"):