allow adding image with sqfs file from internal store

[finkandreas]

This fixes #15
It does not fail anymore when referencing an internal sqfs file. Additionally it allows to retag by label, i.e. this is equivalent:

uenv image add numbat:24/v1 wombat:24/v1
uenv image add numbat:24/v1 $(uenv --repo=$RP image inspect --format='{sqfs}' wombat/24:v1@arapiles%zen3)

[bcumming]

This test is applied twice (lines 402 and 403). Remove one?

Also, you probably only need to check for the presence of the incolved uenv, i.e. numbat/24:*?

[finkandreas]

I just wanted to be consistent with the tests above, that checked for all images every time.
I don't mind changing this and only check for images that were added at this particular test, It is just inconsistent with the above testing style.

[finkandreas]

addressed in latest commit

[bcumming]

You only need to check for wombat/*?

[finkandreas]

addressed in latest commit

[bcumming]

It isn't clear why these checks are being performed.

Ultimately, we want to perform this step if the source uenv/squashfs is not already in the repository, so it would be good if this changed to:

if (!source_in_repo) {
  ...

Where:

// get a reference to the concrete environment after parsing and validating
const auto& uenv = *env->uenvs.begin();

// check whether the squashfs file is inside the repository
const bool source_in_repo = util::is_child(uenv.second.squashfs_path, repo.path.value());

And we can add a new function to src/util/fs.{h,cpp}:

bool is_child(const std::filesystem::path& child, const std::filesystem::path& parent) {
    auto rel = child.lexically_relative(parent);
    return !rel.empty() && *rel.begin() != "..";
}

[finkandreas]

Addressed in latest commit