## Description
Parallel to #611, in service of removing supergraph config, we'd like to
map all current supergraph config into a generated subgraph. To ensure
that we don't end up with an unfortunate naming collision, I've decided
to refine the definition of a subgraph identifier to exclude any string
starting with `__internal`, which we can then have as our "internal
namespace". This change introduces the new type, and updates the
"unknown_namespace" subgraph to come under this umbrella.
This change should be a no-op to any user who hasn't used a subgraph
name prefixed with `__internal`.
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## Changelog
User-defined subgraphs may no longer be given names that start with
`__internal`, as this will be reserved for internally generated
subgraphs.
- Add a changelog entry (in the "Changelog entry" section below) if the
changes
in this PR have any user-facing impact. See
[changelog
guide](https://github.com/hasura/graphql-engine-mono/wiki/Changelog-Guide).
- If no changelog is required ignore/remove this section and add a
`no-changelog-required` label to the PR.
### Product
_(Select all products this will be available in)_
- [x] community-edition
- [x] cloud
<!-- product : end : DO NOT REMOVE -->
### Type
<!-- See changelog structure:
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_(Select only one. In case of multiple, choose the most appropriate)_
- [ ] highlight
- [ ] enhancement
- [ ] bugfix
- [x] behaviour-change
- [ ] performance-enhancement
- [ ] security-fix
<!-- type : end : DO NOT REMOVE -->
### Changelog entry
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User-defined subgraphs may no longer have names that start with
`__internal`.
<!-- changelog-entry : end : DO NOT REMOVE -->
<!-- changelog : end : DO NOT REMOVE -->
V3_GIT_ORIGIN_REV_ID: a6df1bba7b1f2cda4593654c15d168f25b7134b2
