Make setting known_fail yes also prevent looking for archives

[ryandesign (Ryan Carsten Schmidt)]

Some ports do this (this is from gcc49):

if {${os.platform} eq "darwin" && ([vercmp $xcodeversion 9.0] > 0)} {
    known_fail      yes
    depends_lib
    depends_run
    archive_sites
    pre-fetch {
        ui_error "building ${name} is not supported with Xcode 9 or greater"
        return -code error {unsupported platform}
    }
}

In other words in addition to setting known_fail yes and printing an error message, it clears archive_sites. This is probably intended to stop MacPorts from spending time checking for an archive that we already know does not exist. Maybe MacPorts base could arrange for archive_sites to be cleared automatically when known_fail yes is used, or it could use some other method to prevent checking for the presence of archives.

A separate ticket tracks clearing dependencies.

[ctreleaven (Craig Treleaven)]

See also #59640

[jmroot (Joshua Root)]

Effectively taken care of via #60566.