Simplify nix_file_deps checking

To check that dependencies of `nix_file_deps` are correctly declared,
we copy all declared dependencies into the external repository
directory. If any dependencies were missed, then nix evaluation will
fail.

To test that this really works, remove the

```
    nix_file_deps = ["//tests:pkgname.nix"],
```

line in `@nix-file-deps-test`. I tried writing a failure test for this
using Skylibs `analysistest` framework, but that doesn't seem to work
for failures in external repositories.

Like tweag#76, this likewise closes tweag#74, but in a simpler way.

nixpkgs/nixpkgs.bzl

@@ -39,14 +39,12 @@ def _nixpkgs_local_repository_impl(repository_ctx):
         target = repository_ctx.path("default.nix")
     else:
         target = repository_ctx.path(repository_ctx.attr.nix_file)
-        repository_ctx.symlink(target, target.basename)
+        _cp(repository_ctx, target, target.basename)
 
     # Make "@nixpkgs" (syntactic sugar for "@nixpkgs//:nixpkgs") a valid
     # label for the target Nix file.
     repository_ctx.symlink(target.basename, repository_ctx.name)
 
-    _symlink_nix_file_deps(repository_ctx, repository_ctx.attr.nix_file_deps)
-
 nixpkgs_local_repository = repository_rule(
     implementation = _nixpkgs_local_repository_impl,
     attrs = {
@@ -91,15 +89,17 @@ def _nixpkgs_package_impl(repository_ctx):
     if repository_ctx.attr.nix_file and repository_ctx.attr.nix_file_content:
         fail("Specify one of 'nix_file' or 'nix_file_content', but not both.")
     elif repository_ctx.attr.nix_file:
-        repository_ctx.symlink(repository_ctx.attr.nix_file, "default.nix")
+        _cp(repository_ctx, repository_ctx.path(repository_ctx.attr.nix_file), "default.nix")
     elif repository_ctx.attr.nix_file_content:
         expr_args = ["-E", repository_ctx.attr.nix_file_content]
     elif not repositories:
         fail(strFailureImplicitNixpkgs)
     else:
         expr_args = ["-E", "import <nixpkgs> { config = {}; overlays = []; }"]
 
-    _symlink_nix_file_deps(repository_ctx, repository_ctx.attr.nix_file_deps)
+    for dep in repository_ctx.attr.nix_file_deps:
+        path = repository_ctx.path(dep)
+        _cp(repository_ctx, path, path.basename)
 
     expr_args.extend([
         "-A",
@@ -349,9 +349,5 @@ def _executable_path(repository_ctx, exe_name, extra_msg = ""):
             .format(exe_name, " " + extra_msg if extra_msg else ""))
     return path
 
-def _symlink_nix_file_deps(repository_ctx, deps):
-    """Introduce an artificial dependency with a bogus name on each input."""
-    for dep in deps:
-        components = [c for c in [dep.workspace_root, dep.package, dep.name] if c]
-        link = "/".join(components).replace("_", "_U").replace("/", "_S")
-        repository_ctx.symlink(dep, link)
+def _cp(repository_ctx, src, dest):
+    repository_ctx.template(dest, src, executable = False)