Missing build packages

[fnordahl]

The change of default for using Python and associated tools from the build environment introduced in commit 2482f0f and commit 725d2cb are correct and required for delegating buid-on / run-on decisions to charmcraft.

However, they also introduce a regression on the form of missing build-packages.

Adding the following to a charmcraft.yaml solves it:

    build-packages:
      - virtualenv
      - git

However at this point in time there may be a large estate of charms that currently do not define this.

Furthermore the charmcraft reactive plugin most likely should declare these build-packages as bare minimum default anyway.

Perhaps we need to workaround this issue?