panic when using template.replace from within a module

Not sure if I'm attempting to do something that is not supposed to work but here goes:

With this:
```
#@ load('@ytt:template', t='template')
#@ load('job.lib.yml', 'job')
#@ load('fn/standard.star', 'std')

#@ def name_bundle(name):
#@ return std.suffix(name, 'config')
#@ end
#@
#@ def name_upload_job(name):
#@ return t.replace(job.config(std.suffix(name_bundle(name), 'upload')))
#@ end

- _: #@ name_upload_job(bundle.name)
```
where `job.config` is bound to `default_config`:
```
#@ def default_config(name):
name: #@ name
serial: true
build_log_retention:
builds: 20
days: 2
minimum_succeeded_builds: 1
#@ end
```
It works as expected.

However if I try to move the `t.replace` to the function in the `job` module:
```
#@ load('@ytt:template', t='template')
#@ load('job.lib.yml', 'job')
#@ load('fn/standard.star', 'std')

#@ def name_bundle(name):
#@ return std.suffix(name, 'config')
#@ end
#@
#@ def name_upload_job(name):
#@ return job.config(std.suffix(name_bundle(name), 'upload'))
#@ end

- _: #@ name_upload_job(bundle.name)
```
where `job.config` is bound to `default_config`:
```
#@ def _default_config(name):
name: #@ name
serial: true
build_log_retention:
builds: 20
days: 2
minimum_succeeded_builds: 1
#@ end

#@ def default_config(name):
#@ return t.replace(_default_config(name))
#@ end
```
then I get a panic:
```
ytt: Error:
- runtime error: index out of range [-1] (backtrace: goroutine 1 [running]:
in default_config
job.lib.yml:36 | #@ return t.replace(_default_config(name))
in name_upload_job
nexus-uploader.yml:22 | #@ return job.config(std.suffix(name_bundle(name), 'upload'))
in
nexus-uploader.yml:43 | - _: #@ name_upload_job(bundle.name)

reason:
runtime/debug.Stack(0x917400, 0x966da0, 0xc000318900)
runtime/debug/stack.go:24 +0x9d
github.com/k14s/ytt/pkg/template/core.ErrWrapper.func1.1(0xc0001ae3d8)
github.com/k14s/ytt@/pkg/template/core/errs.go:18 +0x9d
panic(0x966da0, 0xc000318900)
runtime/panic.go:679 +0x1b2
github.com/k14s/ytt/pkg/template.(*CompiledTemplate).TplReplaceNode(0xc0003da000, 0xc0001ce730, 0xc0001863f0, 0xc0004259f0, 0x1, 0x1, 0x0, 0x0, 0x0, 0x30, ...)
github.com/k14s/ytt@/pkg/template/compiled_template.go:285 +0xf4
github.com/k14s/ytt/pkg/template/core.ErrWrapper.func1(0xc0001ce730, 0xc0001863f0, 0xc0004259f0, 0x1, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
github.com/k14s/ytt@/pkg/template/core/errs.go:25 +0xff
go.starlark.net/starlark.(*Builtin).CallInternal(0xc0001863f0, 0xc0001ce730, 0xc0004259f0, 0x1, 0x1, 0x0, 0x0, 0x0, 0xc000080000, 0x7f6b18146008, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/value.go:653 +0x8b
go.starlark.net/starlark.Call(0xc0001ce730, 0xae9160, 0xc0001863f0, 0xc0004259f0, 0x1, 0x1, 0x0, 0x0, 0x0, 0xae9120, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/eval.go:1028 +0x190
go.starlark.net/starlark.(*Function).CallInternal(0xc00038f0c0, 0xc0001ce730, 0xc000425700, 0x1, 0x1, 0x0, 0x0, 0x0, 0xc000080000, 0x7f6b18146008, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/interp.go:305 +0x3ed8
go.starlark.net/starlark.Call(0xc0001ce730, 0xae91e0, 0xc00038f0c0, 0xc000425700, 0x1, 0x1, 0x0, 0x0, 0x0, 0xae9b20, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/eval.go:1028 +0x190
go.starlark.net/starlark.(*Function).CallInternal(0xc0003e9540, 0xc0001ce730, 0xc000425680, 0x1, 0x1, 0x0, 0x0, 0x0, 0xc000080000, 0x7f6b18146008, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/interp.go:305 +0x3ed8
go.starlark.net/starlark.Call(0xc0001ce730, 0xae91e0, 0xc0003e9540, 0xc000425680, 0x1, 0x1, 0x0, 0x0, 0x0, 0xae9ae0, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/eval.go:1028 +0x190
go.starlark.net/starlark.(*Function).CallInternal(0xc000217a80, 0xc0001ce730, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x37, 0x38, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/interp.go:305 +0x3ed8
go.starlark.net/starlark.Call(0xc0001ce730, 0xae91e0, 0xc000217a80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x9, ...)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/eval.go:1028 +0x190
go.starlark.net/starlark.(*Program).Init(0xc00000ead8, 0xc0001ce730, 0xc000187cb0, 0x0, 0x0, 0xc000217540)
go.starlark.net@v0.0.0-20190219202100-4eb76950c5f0/starlark/eval.go:358 +0x96
github.com/k14s/ytt/pkg/template.(*CompiledTemplate).eval(0xc000034f60, 0xc0001ce730, 0xc000187cb0, 0x0, 0x0, 0x0, 0x0, 0x0)
github.com/k14s/ytt@/pkg/template/compiled_template.go:155 +0x247
github.com/k14s/ytt/pkg/template.(*CompiledTemplate).Eval(0xc000034f60, 0xc0001ce730, 0xae04e0, 0xc000186bd0, 0xc0000b4500, 0xc0001ce730, 0xc000187aa0, 0x7155ba, 0x7fffe411ff89)
github.com/k14s/ytt@/pkg/template/compiled_template.go:112 +0x6c8
github.com/k14s/ytt/pkg/workspace.(*TemplateLoader).EvalYAML(0xc000186bd0, 0xc0000b47d0, 0xc0000b47d0, 0xc0000b4500, 0xc00001ef00, 0x9, 0x10, 0x1)
github.com/k14s/ytt@/pkg/workspace/template_loader.go:169 +0x614
github.com/k14s/ytt/pkg/workspace.(*LibraryLoader).eval(0xc0001afad8, 0xc0001c6050, 0x0, 0xc00000e488, 0x1, 0x1, 0xc0001c6050, 0x0, 0x0, 0x1, ...)
github.com/k14s/ytt@/pkg/workspace/library_loader.go:139 +0xa6c
github.com/k14s/ytt/pkg/workspace.(*LibraryLoader).Eval(0xc0001afad8, 0xc0001c6050, 0x0, 0x0, 0xc0001c6050)
github.com/k14s/ytt@/pkg/workspace/library_loader.go:90 +0x50
github.com/k14s/ytt/pkg/cmd/template.(*TemplateOptions).RunWithFiles(0xc0000cf040, 0xc00001e580, 0x9, 0x10, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
github.com/k14s/ytt@/pkg/cmd/template/cmd.go:131 +0x3be
github.com/k14s/ytt/pkg/cmd/template.(*TemplateOptions).Run(0xc0000cf040, 0x0, 0x0)
github.com/k14s/ytt@/pkg/cmd/template/cmd.go:85 +0x288
github.com/k14s/ytt/pkg/cmd/template.NewCmd.func1(0xc00011a500, 0xc00011ea10, 0x0, 0x7, 0x0, 0x0)
github.com/k14s/ytt@/pkg/cmd/template/cmd.go:53 +0x2a
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc00011a500, 0xc00011ea10, 0x0, 0x7, 0x0, 0x0)
github.com/cppforlife/cobrautil@v0.0.0-20180924214100-a39a1714c920/misc.go:25 +0xaf
github.com/spf13/cobra.(*Command).execute(0xc00011a500, 0xc00001e090, 0x7, 0x7, 0xc00011a500, 0xc00001e090)
github.com/spf13/cobra@v0.0.3/command.go:762 +0x460
github.com/spf13/cobra.(*Command).ExecuteC(0xc00011a500, 0xc00011a500, 0xe34340, 0x3)
github.com/spf13/cobra@v0.0.3/command.go:852 +0x2ea
github.com/spf13/cobra.(*Command).Execute(...)
github.com/spf13/cobra@v0.0.3/command.go:800
main.main()
github.com/k14s/ytt@/cmd/ytt/ytt.go:17 +0x103
)
```

Am I trying to do something that isn't supposed to work or is this a bug?

Start with the reproducer in the issue and inspect TplReplaceNode in pkg/template/compiled_template.go around line 285, where the panic occurs. Trace how the module call reaches default_config, then add a regression test covering template.replace from within a module. Done means the example produces the expected YAML or a useful error instead of panicking.

Written by the indexing model from the issue text.