Propagate install_if and provider_priority to APKINDEX

[bugaevc]

Resolves https://github.com/go-gitea/gitea/issues/28704

Example of an entry in the generated APKINDEX file:

C:Q1xCO3H9LTTEbhKt9G1alSC87I56c=
P:hello
V:2.12-r1
A:x86_64
T:The GNU Hello program produces a familiar, friendly greeting
U:https://www.gnu.org/software/hello/
L:GPL-3.0-or-later
S:15403
I:36864
o:hello
m:
t:1705934118
D:so:libc.musl-x86_64.so.1
p:cmd:hello=2.12-r1
i:foobar=1.0 !baz
k:42

the i: and k: entries are new.

[bugaevc]

cc @joniw @mintyhippoxyz

[bugaevc]

Looks like install_if should be a single string and not an array after all: even though it has the same general format as depends/provides, abuild stores those as arrays (one per line), but formats install_if into a single space-separated line. So it makes sense to do just the same in the model. This also sidesteps the need for a migration.

PTAL