Implement package.json's imports field (vercel/turborepo#4895)

### Description

The [imports field](https://nodejs.org/api/packages.html#imports) is
similar to `exports`. While `exports` allows a package to control how
others can import it, `imports` allows it control how it wants to import
other packages.

The majority of the code can be shared with the already-implemented
`ExportsField` and `AliasMap`, with just special parsing logic. Besides
that, I just needed to setup the conditional for imports and some
cleanup.

### Testing Instructions

```bash
RUST_BACKTRACE=1 cargo nextest run -E 'test(snapshot__imports__subpath_imports)'
```

Fixes WEB-50
Pair: #49636
fix vercel/turborepo#4799

---------

Co-authored-by: Tobias Koppers <[email protected]>

crates/turbo-tasks-memory/tests/debug.rs

@@ -3,7 +3,7 @@
 use std::sync::Mutex;
 
 use anyhow::Result;
-use turbo_tasks::{debug::ValueDebug, TaskInput};
+use turbo_tasks::debug::ValueDebug;
 use turbo_tasks_testing::{register, run};
 
 register!();

crates/turbopack-core/src/issue/mod.rs

@@ -1,6 +1,5 @@
 pub mod analyze;
 pub mod code_gen;
-pub mod package_json;
 pub mod resolve;
 pub mod unsupported_module;
 

crates/turbopack-core/src/lib.rs

@@ -16,6 +16,7 @@ pub mod error;
 pub mod ident;
 pub mod introspect;
 pub mod issue;
+pub mod package_json;
 pub mod plugin;
 pub mod proxied_asset;
 pub mod reference;

crates/turbopack-core/src/package_json.rs

@@ -0,0 +1,85 @@
+use std::{fmt::Write, ops::Deref};
+
+use anyhow::Result;
+use serde_json::Value as JsonValue;
+use turbo_tasks::{debug::ValueDebugFormat, primitives::StringVc, trace::TraceRawVcs};
+use turbo_tasks_fs::{FileContent, FileJsonContent, FileJsonContentReadRef, FileSystemPathVc};
+
+use super::issue::{Issue, IssueVc};
+
+/// PackageJson wraps the parsed JSON content of a `package.json` file. The
+/// wrapper is necessary so that we can reference the [FileJsonContent]'s inner
+/// [serde_json::Value] without cloning it.
+#[derive(PartialEq, Eq, ValueDebugFormat, TraceRawVcs)]
+pub struct PackageJson(FileJsonContentReadRef);
+
+impl Deref for PackageJson {
+    type Target = JsonValue;
+    fn deref(&self) -> &Self::Target {
+        match &*self.0 {
+            FileJsonContent::Content(json) => json,
+            _ => unreachable!("PackageJson is guaranteed to hold Content"),
+        }
+    }
+}
+
+#[turbo_tasks::value(transparent, serialization = "none")]
+pub struct OptionPackageJson(Option<PackageJson>);
+
+/// Reads a package.json file (if it exists). If the file is unparseable, it
+/// emits a useful [Issue] pointing to the invalid location.
+#[turbo_tasks::function]
+pub async fn read_package_json(path: FileSystemPathVc) -> Result<OptionPackageJsonVc> {
+    let read = path.read_json().await?;
+    match &*read {
+        FileJsonContent::Content(_) => Ok(OptionPackageJson(Some(PackageJson(read))).cell()),
+        FileJsonContent::NotFound => Ok(OptionPackageJson(None).cell()),
+        FileJsonContent::Unparseable(e) => {
+            let mut message = "package.json is not parseable: invalid JSON: ".to_string();
+            if let FileContent::Content(content) = &*path.read().await? {
+                let text = content.content().to_str()?;
+                e.write_with_content(&mut message, &text)?;
+            } else {
+                write!(message, "{}", e)?;
+            }
+            PackageJsonIssue {
+                error_message: message,
+                path,
+            }
+            .cell()
+            .as_issue()
+            .emit();
+            Ok(OptionPackageJson(None).cell())
+        }
+    }
+}
+
+/// Reusable Issue struct representing any problem with a `package.json`
+#[turbo_tasks::value(shared)]
+pub struct PackageJsonIssue {
+    pub path: FileSystemPathVc,
+    pub error_message: String,
+}
+
+#[turbo_tasks::value_impl]
+impl Issue for PackageJsonIssue {
+    #[turbo_tasks::function]
+    fn title(&self) -> StringVc {
+        StringVc::cell("Error parsing package.json file".to_string())
+    }
+
+    #[turbo_tasks::function]
+    fn category(&self) -> StringVc {
+        StringVc::cell("parse".to_string())
+    }
+
+    #[turbo_tasks::function]
+    fn context(&self) -> FileSystemPathVc {
+        self.path
+    }
+
+    #[turbo_tasks::function]
+    fn description(&self) -> StringVc {
+        StringVc::cell(self.error_message.clone())
+    }
+}