Properly support build-snaps with differing versions for independent parts.

[aznashwan]

What needs to get done

Make rockcraft automatically detect when parts need different versions of the same build-snaps and either snap refresh said snaps between the parts, or at the very least give the user a clear warning it's not going to work as expected.

Why it needs to get done

Having two separate parts needing differing versions of the same build-snaps during the build stage is very reasonable.

The canonical/trivy-adapter ROCK for example builds both trivy and a separate trivy-adapter-photon binary with different Go versions.

You can reproduce the issue using this sample file:

# cat rockcraft.yaml 
name: go-snap-parts-version-test
summary: Test image to showcase snap parts issue.
description: Test image to showcase snap parts issue.

version: v1.0.0
license: Apache-2.0

base: [email protected]
build-base: [email protected]
platforms:
  amd64:
  # arm64:

services:
  sleep:
    command: sleep inf
    override: replace
    startup: enabled

parts:

  go118:
    plugin: go
    build-snaps:
      - go/1.18/stable
    source: https://github.com/canonical/rockcraft
    source-type: git
    source-tag: 1.5.3
    source-depth: 1

    override-build: |
      go version

  go121:
    plugin: go
    build-snaps:
      - go/1.21/stable
    source: https://github.com/canonical/rockcraft
    source-type: git
    source-tag: 1.5.3
    source-depth: 1

    override-build: |
      go version

Running rockcraft pack yields the following:

$ rockcraft pack --verbose --debug
Starting rockcraft, version 1.5.3                                                                                                                                                
[...]
Building go118                                                                                                                                                                   
:: + go version                                                                                                                                                                  
:: go version go1.18.10 linux/amd64                                                                                                                                              
Building go121                                                                                                                                                                   
:: + go version                                                                                                                                                                  
:: go version go1.18.10 linux/amd64                                                                                                                                              
[...] 

The only current solution available is manually snap refresh-ing the packages in override-build, which is not ideal...