go_path: support go:embed of generated files

When embedded files are generated, relativize fails because the file
path starts with the output directory. Without the change in path.bzl,
the test case fails as follows:

Error in fail: Path 'bazel-out/darwin-fastbuild/bin/tests/core/go_path/pkg/lib/renamed_embedded_src.txt'
is not beneath 'tests/core/go_path/pkg/lib'

go/private/tools/path.bzl

@@ -83,7 +83,8 @@ def _go_path_impl(ctx):
         for f in pkg.embedsrcs:
             if src_dir == None:
                 fail("cannot relativize {}: src_dir is unset".format(f.path))
-            dst = pkg.dir + "/" + paths.relativize(f.path, src_dir)
+            embedpath = paths.relativize(f.path, f.root.path)
+            dst = pkg.dir + "/" + paths.relativize(embedpath, src_dir)
             _add_manifest_entry(manifest_entries, manifest_entry_map, inputs, f, dst)
     if ctx.attr.include_pkg:
         for pkg in pkg_map.values():

tests/core/go_path/BUILD.bazel

@@ -18,6 +18,15 @@ test_suite(name = "go_path")
     ],
 ) for mode in ("archive", "copy", "link")]
 
+go_path(
+    name = "transition_path",
+    testonly = True,
+    data = ["extra.txt"],
+    include_pkg = True,
+    mode = "copy",
+    deps = ["//tests/core/go_path/cmd/bin:pie"],
+)
+
 go_path(
     name = "nodata_path",
     testonly = True,
@@ -49,6 +58,7 @@ go_test(
         ":archive_path",
         ":copy_path",
         ":link_path",
+        ":transition_path",
         ":nodata_path",
         ":notransitive_path",
     ],

tests/core/go_path/cmd/bin/BUILD.bazel

@@ -8,6 +8,17 @@ go_binary(
     visibility = ["//visibility:public"],
 )
 
+go_binary(
+    name = "pie",
+    srcs = ["bin.go"],
+    importpath = "example.com/repo/cmd/bin",
+    visibility = ["//visibility:public"],
+    linkmode="pie",
+    deps = [
+        "//tests/core/go_path/pkg/lib:go_default_library"
+    ]
+)
+
 go_binary(
     name = "cross",
     srcs = ["bin.go"],

tests/core/go_path/pkg/lib/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+load("@bazel_skylib//rules:copy_file.bzl", "copy_file")
 
 go_library(
     name = "go_default_library",
@@ -10,13 +11,20 @@ go_library(
     ],
     embedsrcs = [
         "embedded_src.txt",
+        "renamed_embedded_src.txt",
         "template/index.html.tmpl",
     ],
     importpath = "example.com/repo/pkg/lib",
     visibility = ["//visibility:public"],
     deps = [":transitive_lib"],
 )
 
+copy_file(
+    name = "rename_embedded_src.txt",
+    src = ":embedded_src.txt",
+    out = "renamed_embedded_src.txt",
+)
+
 go_test(
     name = "go_default_test",
     srcs = [

tests/core/go_path/pkg/lib/lib.go

@@ -8,5 +8,8 @@ import (
 //go:embed embedded_src.txt
 var embeddedSource string
 
+//go:embed renamed_embedded_src.txt
+var renamedEmbeddedSource string
+
 //go:embed template/index.html.tmpl
 var indexTmpl string