many: differentiate between "distro" and "core" libexecdir

[zyga]

Inside the core snap libexecdir is always /usr/lib/snapd but on the outside it
may be also /usr/libexec/snapd. In order for snap-confine and snap-exec to play
ball across the root filesystem transition we need to use the right path.

Signed-off-by: Zygmunt Krynicki me@zygoon.pl

[Conan-Kudo]

I feel like this PR should be closed, since its commit is part of #2845...

[zyga]

@Conan-Kudo this is a github limitation. I specifically indicated in the other PR that it is stacked on top of this one. This branch can be reviewed and landed and reviews of the other branch should focus on the (stacked) new commit. Closing it would just focus reviewers to see a larger diff. I think this way the code is easier to track.

[Conan-Kudo]

LGTM.

[vosst]

LGTM