store: change main store host to api.snapcraft.io

[cjwatson]

As part of the enterprise store project, the store is moving over to
having a single api.snapcraft.io host which should eventually be the
front door for all queries from devices; this will make it much easier
to put something like an edge proxy in place, because only one URL will
need to be overridden.

At the moment, api.snapcraft.io only provides the non-legacy endpoints
that used to be part of Click Package Index, but it will soon be
expanded to provide others as well. This shifts snapd over to the new
host name. The older endpoints will of course be maintained for as long
as is necessary.

I've dropped the trailing api/v1/ from the URL, since we'll probably
have a v2/ at some point but that may be on an endpoint-by-endpoint
basis, so it doesn't make sense to pin all of snapd's API calls to the
same version.

The preferred way to set an alternate store API URL is now
SNAPPY_FORCE_API_URL=https://api.example.org/.
SNAPPY_USE_STAGING_STORE=1 still works as an abbreviated way to point
at staging.

[codecov-io]

Codecov Report

Merging #3475 into master will increase coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #3475      +/-   ##
==========================================
+ Coverage   77.15%   77.15%   +<.01%     
==========================================
  Files         373      375       +2     
  Lines       25793    25839      +46     
==========================================
+ Hits        19901    19937      +36     
- Misses       4134     4140       +6     
- Partials     1758     1762       +4

Impacted Files	Coverage Δ
tests/lib/fakestore/store/store.go	64.05% <100%> (ø)	⬆️
store/store.go	79.44% <72.72%> (-0.18%)	⬇️
interfaces/sorting.go	90% <0%> (-3.34%)	⬇️
cmd/snap/cmd_aliases.go	94% <0%> (-2%)	⬇️
dirs/dirs.go	96.77% <0%> (-0.06%)	⬇️
daemon/api.go	73.18% <0%> (-0.01%)	⬇️
interfaces/builtin/udisks2.go	82.5% <0%> (ø)	⬆️
interfaces/builtin/content.go	80.73% <0%> (ø)	⬆️
interfaces/policy/basedeclaration.go	40.74% <0%> (ø)	⬆️
client/asserts.go	87.5% <0%> (ø)	⬆️
... and 6 more

---

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[mvo5]

Thanks Colin, this looks great.

[pedronis]

lgtm, couple small remarks

[pedronis]

we probably want a TODO: or XXX: marker here, I suppose we'll drop this at some point

[pedronis]

this maybe could use strings.TrimSuffix

[zyga]

Looks good