Fixing precedence issue with the import of values.

[mattfarina]

The ordering should be:

1. User specified values (e.g CLI)
2. Parent chart values
3. Imported values
4. Sub-chart values

This enables parent charts to import large set of values from a child and then override select values.

This change is needed for backwards compatibility.

Fixes #12460

What this PR does / why we need it: The values fixes in 3.13.0 had the wrong prioritization for values which broke backwards compatibility and did not make sense. Imported values should be able to be used (e.g., with a library chart) while being able override select values.

Special notes for your reviewer:

• Tests that change here are restored to 3.12.x values for the tests.
• This restores the previous ordering. See 3.12.3 here.

If applicable:

• this PR contains documentation
• this PR contains unit tests
• this PR has been tested for backwards compatibility

[gjenkins8]

LGTMe.

Tested this change against the example in #12460, looks good to me (it was also required to update the example chart's values image: tag: "" to be "latest" to produce valid output yaml; same as for Helm 3.12).

[joejulian]

Passes every test I have to throw at it.

[zegerius]

This also fixes our regression, tested with https://github.com/ankitabhopatkar13/helm-values-issue.

$ diff <(/opt/homebrew/Cellar/helm/3.13.0/bin/helm upgrade --install umbrella umbrella --dry-run --values umbrella/values.yaml) <(/usr/local/bin/helm upgrade --install umbrella umbrella --dry-run --values umbrella/values.yaml)
25c25
<     - port: 9090
---
>     - port: 1234
47c47
<     - port: 9090
---
>     - port: 8080

and the diff between 3.12.3 and this PR is zero!

$ diff <(/opt/homebrew/Cellar/helm/3.12.3/bin/helm upgrade --install umbrella umbrella --dry-run --values umbrella/values.yaml) <(/usr/local/bin/helm upgrade --install umbrella umbrella --dry-run --values umbrella/values.yaml)

[intetinte]

This change has a flaw in the sense that the values defined in a sub-chart will not get the values from an import. The import only populates non-existing data in the sub-chart values.

Example:

exports:
sub-chart:
A:
A: hello
B: good
C: world
D: sunshine

Sub-chart values:
A:
A: null
B: bad
D:

Result:
A:
A:
B: bad
C: world
D:

Can add that the current order of precedence basically is:

1. User specified values (e.g CLI)
2. Sub-chart values
3. Imported values
4. Parent chart values