Package system improvements

[ ] Implement target (component) selection (migrate from the concept of goal components)
- [ ] Integrate into the documentation generator
[ ] Implement private (secondary) components (via the manifest entry public: <true|false>)
[ ] Implement the test packages/transitive-dependencies-are-inaccessible
[ ] Add several tests to cover all three uses of Diagnostic::undefined_library_component
- [ ] An attempt to depend on a non-library component should lead to a better error message (e.g. add a note).
[ ] Writing a: { b: { path: "." } } should lead to a good error message
[ ] Add tests for the package entry in dependency declarations
[ ] “Circular packages” (circular packages from distinct packages) should lead to all offending components to be reported at once (via primary highlights). Currently, only the first violating package is reported. Requires a bit of an architectural redesign I guess. See test packages/circular-components-three-packages
[ ] Improve the test packages/circular-components-in-same-package (re. “outsiders” and “unrelated components”)
[ ] Parallel package resolution
[ ] Implement the dependency provider git (git dependencies)
[ ] Initialize an empty git repo on lushui <new|init> unless --vsc=none (create the latter flag as well)
[ ] Actually parse versions as Cargo-flavored SemVer versions
[ ] Implement the dependency provider registry (HTTP(s) registry dependencies)
- [ ] Enforce and use versions
[ ] Support for dependencies for single-file packages via a CLI argument --extern=NAME:PATH
[ ] Option to set the path to the folder of distributed packages via CLI argument --distributed-libraries=PATH (or hidden behind a new option group -Y=distributed-libraries:PATH (…)) (should this be called the sysroot or a variation thereof?)
[x] Better error messages and span information for the package resolver
- [x] circular components (could be further improved) (via e768af3b88aa924fe4ab9aa7dd12115976f7c502)
- [x] dependency is not a library
- [x] metadata parsing errors
- [x] loading errors (could be further improved) (via e768af3b88aa924fe4ab9aa7dd12115976f7c502)
[x] UI test suite for packages (not just single-file components/packages) (via 83c7b62e536bb065e1657096bbef00f73016e204)
[x] More robust and correct handling of “intrinsics”/“lang items” (foreign bindings, not C FFI) (via 2de1c9b71141f76173f3edb020971978104d873c)
[x] Better metadata deserialization API (via 2360544279fb1676f88dedc3093b0c90f714fb4a)
[x] ~~More intelligent behavior for -Z emit-<data> and -Z <pass>-only: Options for dumping info of every crate or only the goal crate; options for phase restrictions in presence of multiple crates~~ (postponed indefinitely; current behavior emits data for target components)
[x] lushui <check|build|run> <path/to/package-folder> should check/build/run a package at the given path; checking/building/running a file should continue to work (via 8db91a93dd05e043cbd4c554407a45f0b5ce74df)
[x] Support lushui <check|build|run> <path/to/name.metadata> for some name but for the fixed extension metadata should check/build/run the package corresp. to the given manifest or lushui <check|build|run> --manifest <path/to/manifest> (the latter is more robust)

fmease / lushui

Package system improvements #100