Promote package suggestions to a first-class concept on IndexedDocsProviders (#16177)

This PR promotes package suggestions to a first-class concept on the
`IndexedDocsProvider` trait.

This will allow any implementer of `IndexedDocsProvider` to provide a
list of package names to suggest for use with `/docs`.

For the docs.rs provider we use the 250 most popular Rust crates (as
identified [here](https://lib.rs/std)), and for the rustdoc provider we
use the packages in the Cargo workspace.

Release Notes:

- N/A

crates/indexed_docs/src/providers/rustdoc/popular_crates.txt

@@ -0,0 +1,252 @@
+# A list of the most popular Rust crates.
+# Sourced from https://lib.rs/std.
+serde
+serde_json
+syn
+clap
+thiserror
+rand
+log
+tokio
+anyhow
+regex
+quote
+proc-macro2
+base64
+itertools
+chrono
+lazy_static
+once_cell
+libc
+reqwest
+futures
+bitflags
+tracing
+url
+bytes
+toml
+tempfile
+uuid
+indexmap
+env_logger
+num-traits
+async-trait
+sha2
+hex
+tracing-subscriber
+http
+parking_lot
+cfg-if
+futures-util
+cc
+hashbrown
+rayon
+hyper
+getrandom
+semver
+strum
+flate2
+tokio-util
+smallvec
+criterion
+paste
+heck
+rand_core
+nom
+rustls
+nix
+glob
+time
+byteorder
+strum_macros
+serde_yaml
+wasm-bindgen
+ahash
+either
+num_cpus
+rand_chacha
+prost
+percent-encoding
+pin-project-lite
+tokio-stream
+bincode
+walkdir
+bindgen
+axum
+windows-sys
+futures-core
+ring
+digest
+num-bigint
+rustls-pemfile
+serde_with
+crossbeam-channel
+tokio-rustls
+hmac
+fastrand
+dirs
+zeroize
+socket2
+pin-project
+tower
+derive_more
+memchr
+toml_edit
+static_assertions
+pretty_assertions
+js-sys
+convert_case
+unicode-width
+pkg-config
+itoa
+colored
+rustc-hash
+darling
+mime
+web-sys
+image
+bytemuck
+which
+sha1
+dashmap
+arrayvec
+fnv
+tonic
+humantime
+libloading
+winapi
+rustc_version
+http-body
+indoc
+num
+home
+serde_urlencoded
+http-body-util
+unicode-segmentation
+num-integer
+webpki-roots
+phf
+futures-channel
+indicatif
+petgraph
+ordered-float
+strsim
+zstd
+console
+encoding_rs
+wasm-bindgen-futures
+urlencoding
+subtle
+crc32fast
+slab
+rustix
+predicates
+spin
+hyper-rustls
+backtrace
+rustversion
+mio
+scopeguard
+proc-macro-error
+hyper-util
+ryu
+prost-types
+textwrap
+memmap2
+zip
+zerocopy
+generic-array
+tar
+pyo3
+async-stream
+quick-xml
+memoffset
+csv
+crossterm
+windows
+num_enum
+tokio-tungstenite
+crossbeam-utils
+async-channel
+lru
+aes
+futures-lite
+tracing-core
+prettyplease
+httparse
+serde_bytes
+tracing-log
+tower-service
+cargo_metadata
+pest
+mime_guess
+tower-http
+data-encoding
+native-tls
+prost-build
+proptest
+derivative
+serial_test
+libm
+half
+futures-io
+bitvec
+rustls-native-certs
+ureq
+object
+anstyle
+tonic-build
+form_urlencoded
+num-derive
+pest_derive
+schemars
+proc-macro-crate
+rstest
+futures-executor
+assert_cmd
+termcolor
+serde_repr
+ctrlc
+sha3
+clap_complete
+flume
+mockall
+ipnet
+aho-corasick
+atty
+signal-hook
+async-std
+filetime
+num-complex
+opentelemetry
+cmake
+arc-swap
+derive_builder
+async-recursion
+dyn-clone
+bumpalo
+fs_extra
+git2
+sysinfo
+shlex
+instant
+approx
+rmp-serde
+rand_distr
+rustls-pki-types
+maplit
+sqlx
+blake3
+hyper-tls
+dotenvy
+jsonwebtoken
+openssl-sys
+crossbeam
+camino
+winreg
+config
+rsa
+bit-vec
+chrono-tz
+async-lock
+bstr