kpt 1.0 can't use blueprint retrieved via kpt 0.39

[martinmaly]

Expected behavior

kpt 0.39 should not downgrade blueprints to become incompatible with kpt 1.0.

Options:

• return an error
• make the pkg get code forward-compatible so it can detect kpt 1.0 package and preserve its structure

note that customer needs to fetch the package before they can see which version of kpt it is compatible with. Ideally we would allow:

• fetch package using any kpt
• check the compatibility of the package (i.e. "kpt tell me if this is v0.39 or v1 package)
• treat it accordingly

Actual behavior

Package is downgraded and kpt 1.0 cannot work with it.

Information

Steps to reproduce the behavior

% kpt version
0.39.2

% kpt pkg get https://github.com/GoogleContainerTools/kpt/package-examples/nginx@v0.4 nginx-039
fetching package "package-examples/nginx" from "https://github.com/GoogleContainerTools/kpt" to "nginx-039"

% kpt1 version
1.0.0-alpha.4
% kpt1 fn render nginx-039 
Error: Kptfile at "nginx-039" can't be read.

Details:
yaml: unmarshal errors:
  line 8: field commit not found in type v1alpha2.Git 

% kpt1 pkg get https://github.com/GoogleContainerTools/kpt/package-examples/nginx@v0.4 nginx-1  
Package "nginx-1":
Fetching https://github.com/GoogleContainerTools/kpt@v0.4.
From https://github.com/GoogleContainerTools/kpt
 * tag                 package-examples/nginx/v0.4 -> FETCH_HEAD
Adding package "package-examples/nginx".
kpt1 fn
Fetched 1 package(s).

% kpt1 fn render nginx-1 
Package "nginx-1": 
[RUNNING] "gcr.io/kpt-fn/kubeval:v0.1"
[PASS] "gcr.io/kpt-fn/kubeval:v0.1"

Successfully executed 1 function(s) in 1 package(s).

[mortent]

We are releasing a new version of pre 1.0 kpt (v0.39.3) that will provide better error messages when encountering packages where the Kptfile is using a new schema. Fixed in #2182

We have also added a better error message when using kpt v1.0 to fetch a package with the old Kptfile schema. Automatically updating the package is not a good solution here since there are substantial differences between the old and the new schema. Fixed in #2161