Create PackageCollection type for nicer show method

[ericphanson]

We could return a stuct instead of a raw Vector{PackageV1} from functions which return multiple packages so we could add a custom show method to surface info. Here is a quick version to see.

julia> using PackageAnalyzer

julia> analyze_manifest()
57-element PackageCollection
* Source code
  * Total Julia source code: 51_733 lines
  * Total Julia extension code: 257 lines (0.3% of `test` + `src` + `ext`)
  * Julia code test code: 36_066 lines (41.0% of `test` + `src` + `ext`)
  * Lines of documentation: 5_686 lines (9.9% of `docs` + `src` + `ext`)
  * Total lines of documentation in README & docstrings: 14862 lines (22.3% of README + `src`)
* Licenses
  * MIT (54): AbstractTrees, BitFlags, BitIntegers, CodecLz4, CodecZlib, CodecZstd, Compat, ConcurrentUtilities, DataAPI, DataValueInterfaces, EnumX, ExceptionUnwrapping, Expat_jll, ExprTools, Git, GitHub, Git_jll, HTTP, InlineStrings, IteratorInterfaceExtensions, JLLWrappers, JSON, JSON3, JuliaSyntax, LazilyInitializedFields, Legolas, Libiconv_jll, LicenseCheck, LoggingExtras, Lz4_jll, MbedTLS, Mocking, OpenSSL, OpenSSL_jll, OrderedCollections, Parsers, PooledArrays, PrecompileTools, Preferences, RegistryInstances, Scratch, SentinelArrays, SodiumSeal, StructTypes, TZJData, TableTraits, Tables, TimeZones, Tokei_jll, TranscodingStreams, URIs, Zstd_jll, libsodium_jll, licensecheck_jll
  * Apache-2.0 (3): Arrow, ArrowTypes, MbedTLS

I've subtyped AbstractVector so most of the same things still work as before. Nonetheless it's probably breaking since we documented the return type.

cc @tecosaur who probably knows how to make it look a lot nicer :)