Extract "pkg/version".PrintAndExitIfRequested() to its own package

because it causes a runtime panic if a binary which has its own implementation
of "-version" flag tries to reuse a package library which indirectly depend on
"pkg/version".

e.g. If such an user-defined binary tires to link "pkg/api" or "pkg/client",
the binary fails with a runtime panic "flag redefined: version".