puffin-resolver: make VersionMap construction lazy
That is, a `PrioritizedDistribution` for a specific version of a
package is not actually materialized in memory until a corresponding
`VersionMap::get` call is made for that version. Similarly, iteration
lazily materializes distributions as it moves through the map. It
specifically does not materialize everything first.
The main reason why this is effective is that an
`OwnedArchive<SimpleMetadata>` represents a zero-copy (other than
reading the source file) version of `SimpleMetadata` that is really just
a `Vec<u8>` internally. The problem with `VersionMap` construction
previously is that it had to eagerly materialize a `SimpleMetadata` in
memory before anything else, which defeats a large part of the purpose
of zero-copy deserialization. By making more of `VersionMap`
construction itself lazy, we permit doing some parts of resolution
without necessarily fully deserializing a `SimpleMetadata` into memory.
Indeed, with this commit, in the warm cached case, a `SimpleMetadata` is
itself never materialized fully in memory.
This does not completely and totally fully realize the benefits of
zero-copy deserialization. For example, we are likely still building
lots of distributions in memory that we don't actually need in some
cases. Perhaps in cases where no resolution exists, or when one needs to
iterate over large portions of the total versions published for a
package.
