Better sysreqs support for manylinux packages

[gaborcsardi]

E.g. do not try to install system packages for them.

[hadley]

It's probably too soon to default to manylinux, but we might want to fall back to it if there's not a more specific P3M binary available.

[gaborcsardi]

It's probably too soon to default to manylinux, but we might want to fall back to it if there's not a more specific P3M binary available.

I don't expect that this would be common, because manylinux packages are typically harder to build.

Nevertheless if you configure both repos and there is no distro-specific binary, then pak will install the manylinux binary already. OTOH it might also install the manylinux binary if there is a disto-specific one available.

But in general I don't expect that people would configure both repositories at the same time.