many: support the use of build-aux/snap

[sergiusens]

Many projects required an alternate snap directory to store
snapcraft related assets, in particular Gnome relate projects
have consolidated with the use of build-aux which snapcraft
is now adopting as an alternative as well.

LP: #1805219

Signed-off-by: Sergio Schvezov sergio.schvezov@canonical.com

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh tests/unit?

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@47141bf). Click here to learn what that means.
The diff coverage is 86.66%.

@@            Coverage Diff            @@
##             master    #2496   +/-   ##
=========================================
  Coverage          ?   89.91%           
=========================================
  Files             ?      198           
  Lines             ?    13360           
  Branches          ?     2022           
=========================================
  Hits              ?    12013           
  Misses            ?      922           
  Partials          ?      425

Impacted Files	Coverage Δ
snapcraft/project/_get_snapcraft.py	83.33% <ø> (ø)
snapcraft/project/_project.py	92% <100%> (ø)
snapcraft/internal/meta/_snap_packaging.py	91.08% <100%> (ø)
snapcraft/cli/lifecycle.py	76.02% <60%> (ø)

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