internal/common: added PgidSet to replace usages of map[Pgid]bool

[arturmelanchyk]

As a followup of the work carried out in #1082.

In the freelist package I found pidSet that, if extended to have Add, Has, Remove methods and made public, can replace all usages of map[Pgid]bool and map[Pgid]struct{}
Let me know if this new PgidSet type looks good to you or if you prefer to stick to using map[string]struct{} instead.

[k8s-ci-robot]

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