charm version compatibility output is unfortunately not compatible with charmcraft 2.0

[fnordahl]

The code in charmcraft 2.0 looks like this:

            output = self._execute("charm version").strip()
            _, tools_version = output.split("\n")

And our current compatibility code look like this:

charm-tools/charmtools/version.py

Lines 70 to 75 in e1c7ee0

	# ensure compatibility with anyone parsing this output expecting the
	# charm-tools version on the second line ref:
	# https://github.com/canonical/charmcraft/blob/dbf82a0fb/charmcraft/reactive_plugin.py#L65
	print("")
	print("charm-tools {}".format(
	format_version(cached_charm_tools_version(), args.format)))

Due to charmcraft's use of strip as opposed to rstrip, this does not work.

Example:

>>> s = subprocess.run(["/bin/bash", "-c", "charm version"], check=True, capture_output=True, universal_newlines=True)
>>> s
CompletedProcess(args=['/bin/bash', '-c', 'charm version'], returncode=0, stdout='\ncharm-tools 3.0.0+snap-661+git-1-b7025b2\n', stderr='')
>>> s.stdout.strip()
'charm-tools 3.0.0+snap-661+git-1-b7025b2'

Now if we fudge the output by adding something on the first line:

>>> s = subprocess.run(["/bin/bash", "-c", "charm version"], check=True, capture_output=True, universal_newlines=True)
>>> s
CompletedProcess(args=['/bin/bash', '-c', 'charm version'], returncode=0, stdout='-\ncharm-tools 3.0.0+snap-x1+git-3-e1c7ee0\n', stderr='')
>>> s.stdout.strip()
'-\ncharm-tools 3.0.0+snap-x1+git-3-e1c7ee0'
>>> _, tools_version = s.stdout.strip().split('\n')
>>> tools_version
'charm-tools 3.0.0+snap-x1+git-3-e1c7ee0'