Synthesize fake GOPATH and remove bazel_copy_generated_for_ide.sh

We can use rules_go to create a fake symlink farm GOPATH. This can be
set as the GOPATH in IDEs that do not have native support for Bazel,
and we can stop maintaining the bazel_copy_generated_for_ide.sh hack.

The remaining issue is that we still need Go tooling to manage our
go.mod file that serves as the input to Gazelle, so we need to add
replace directives to ensure that Go tools can resolve generated code.

The proper fix for all this is the upcoming native Bazel support in Go:
bazel-contrib/rules_go#512

Test Plan:
Ran `bazel build :gopath`, and a wild
`bazel-bin/gopath/src/git.monogon.dev/source/nexantic.git/core/generated/api/schema.pb.go`
appeared.

`scripts/symlink_generated_files.sh` created a valid symlink in core/generated
and `go mod tidy` ran successfully (despite complaining about the symlink).

Running `scripts/gazelle.sh` twice worked.

X-Origin-Diff: phab/D305
GitOrigin-RevId: 0d456bc57d4a2d72e30865ffef777d2f5be5c407

BUILD

@@ -1,5 +1,5 @@
 load("@bazel_gazelle//:def.bzl", "gazelle")
-load("@io_bazel_rules_go//go:def.bzl", "nogo")
+load("@io_bazel_rules_go//go:def.bzl", "go_path", "nogo")
 
 # gazelle:prefix git.monogon.dev/source/nexantic.git
 # gazelle:exclude core/generated
@@ -55,3 +55,12 @@ nogo(
         "@org_golang_x_tools//go/analysis/passes/unusedresult:go_tool_library",
     ],
 )
+
+# Synthesize a fake GOPATH in bazel-bin/gopath for IDEs without Bazel support
+go_path(
+    name = "gopath",
+    mode = "link",
+    deps = [
+        "//core/cmd/init",
+    ],
+)

scripts/gazelle.sh

@@ -2,6 +2,8 @@
 # gazelle.sh regenerates BUILD.bazel files for Go source files.
 set -euo pipefail
 
+scripts/symlink_generated_files.sh
+
 bazel run //:go mod tidy
 bazel run //:gazelle -- update
 bazel run //:gazelle -- update-repos -from_file=go.mod -to_macro=repositories.bzl%go_repositories -prune=true

scripts/symlink_generated_files.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+create_symlinks() {
+  local package=$1
+
+  rm -rf "$package"
+  ln -r -s "bazel-bin/gopath/src/git.monogon.dev/source/nexantic.git/$package" "$package"
+}
+
+create_symlinks core/generated