Allow config entries in Pulumi.<stack>.yaml without a namespace

[tedchang77]

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Issue details

Configuration entries in Pulumi.stack.yaml require you to namespace all configurations. This is a challenge if you have to deploy the same application with a different project name, see #8537, as you need to modify the namespace for each configuration. you also end up duplicating the namespace text for many configurations, which can be repetitive and error prone.

Currently when you set configuration on the CLI or read configurations from the Config API you don't need to set the namespace in the configuration as it uses the project name defined in Pulumi.yaml file.

CLI:
pulumi config set key-with-no-namespace value

API:
let config = new pulumi.Config(); // namespace defaults to project name if its not set

can we also support inferring the namespace from the Pulumi.yaml project name if the namespace is not provided in the configuration key? so the configs in Pulumi.stack.yaml would look like this:

config:
  replicas: 1

instead of:

config:
  project-name:replicas: 1

Affected area/feature

pulumi CLI
pulumi Config API

[t0yv0]

Related #2307

[antdking]

Additionally, comments + whitespace get stripped when it changes unprefixed to prefixed keys (ie, round-tripping #11456 is broken)

I'm not using it for DRY reasons, it just looks cleaner.