template.replace() can't replace a map item with the same key

**What steps did you take:**
```yaml
#@ load("@ytt:template", "template")
_: #@ template.replace({"_": "1"})
```

**What happened:**
```console
ytt: Error:
- template.replace: expected to find placeholder map item in map
in
ex2.yaml:2 | _: #@ template.replace({"_": "1"})
```

**What did you expect:**
The map item to simply be replaced.

**Anything else you would like to add:**
[#carvel @ 2022-07-11](https://kubernetes.slack.com/archives/CH8KCCKA5/p1657433680791589)

**Environment:**

- ytt version (use `ytt --version`):
- OS (e.g. from `/etc/os-release`):

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start by reproducing the template.replace example from ex2.yaml with ytt and trace the template.replace entry point that reports the missing placeholder map item. Confirm the map item is replaced when the placeholder and replacement use the same key, while preserving the existing error behavior for genuinely missing placeholders.

Written by the indexing model from the issue text.