Source dist metadata refactor (#468)

## Summary and motivation

For a given source dist, we store the metadata of each wheel built
through it in `built-wheel-metadata-v0/pypi/<source dist
filename>/metadata.json`. During resolution, we check the cache status
of the source dist. If it is fresh, we check `metadata.json` for a
matching wheel. If there is one we use that metadata, if there isn't, we
build one. If the source is stale, we build a wheel and override
`metadata.json` with that single wheel. This PR thereby ties the local
built wheel metadata cache to the freshness of the remote source dist.
This functionality is available through `SourceDistCachedBuilder`.

`puffin_installer::Builder`, `puffin_installer::Downloader` and
`Fetcher` are removed, instead there are now `FetchAndBuild` which calls
into the also new `SourceDistCachedBuilder`. `FetchAndBuild` is the new
main high-level abstraction: It spawns parallel fetching/building, for
wheel metadata it calls into the registry client, for wheel files it
fetches them, for source dists it calls `SourceDistCachedBuilder`. It
handles locks around builds, and newly added also inter-process file
locking for git operations.

Fetching and building source distributions now happens in parallel in
`pip-sync`, i.e. we don't have to wait for the largest wheel to be
downloaded to start building source distributions.

In a follow-up PR, I'll also clear built wheels when they've become
stale.

Another effect is that in a fully cached resolution, we need neither zip
reading nor email parsing.

Closes #473

## Source dist cache structure 

Entries by supported sources:
 * `<build wheel metadata cache>/pypi/foo-1.0.0.zip/metadata.json`
* `<build wheel metadata
cache>/<sha256(index-url)>/foo-1.0.0.zip/metadata.json`
* `<build wheel metadata
cache>/url/<sha256(url)>/foo-1.0.0.zip/metadata.json`
But the url filename does not need to be a valid source dist filename

(<https://github.com/search?q=path%3A**%2Frequirements.txt+master.zip&type=code>),
so it could also be the following and we have to take any string as
filename:
* `<build wheel metadata
cache>/url/<sha256(url)>/master.zip/metadata.json`

Example:
```text
# git source dist
pydantic-extra-types @ git+https://github.com/pydantic/pydantic-extra-types.git
# pypi source dist
django_allauth==0.51.0
# url source dist
werkzeug @ ff1904eb5e/werkzeug-3.0.1.tar.gz
```
will be stored as
```text
built-wheel-metadata-v0
├── git
│   └── 5c56bc1c58c34c11
│       └── 843b753e9e8cb74e83cac55598719b39a4d5ef1f
│           └── metadata.json
├── pypi
│   └── django-allauth-0.51.0.tar.gz
│       └── metadata.json
└── url
    └── 6781bd6440ae72c2
        └── werkzeug-3.0.1.tar.gz
            └── metadata.json
```

The inside of a `metadata.json`:
```json
{
  "data": {
    "django_allauth-0.51.0-py3-none-any.whl": {
      "metadata-version": "2.1",
      "name": "django-allauth",
      "version": "0.51.0",
      ...
    }
  }
}
```

crates/puffin-git/src/lib.rs

@@ -1,7 +1,7 @@
 use url::Url;
 
-use crate::git::GitReference;
-pub use crate::source::{GitSource, Reporter};
+use git::GitReference;
+pub use source::{Fetch, GitSource, Reporter};
 
 mod git;
 mod source;

crates/puffin-git/src/source.rs

@@ -1,7 +1,7 @@
 //! Git support is derived from Cargo's implementation.
 //! Cargo is dual-licensed under either Apache 2.0 or MIT, at the user's choice.
 //! Source: <https://github.com/rust-lang/cargo/blob/23eb492cf920ce051abfc56bbaf838514dc8365c/src/cargo/sources/git/source.rs>
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 use anyhow::Result;
 use reqwest::Client;
@@ -120,15 +120,20 @@ pub struct Fetch {
     path: PathBuf,
 }
 
-impl From<Fetch> for GitUrl {
-    fn from(fetch: Fetch) -> Self {
-        fetch.git
+impl Fetch {
+    pub fn git(&self) -> &GitUrl {
+        &self.git
     }
-}
 
-impl From<Fetch> for PathBuf {
-    fn from(fetch: Fetch) -> Self {
-        fetch.path
+    pub fn path(&self) -> &Path {
+        &self.path
+    }
+    pub fn into_git(self) -> GitUrl {
+        self.git
+    }
+
+    pub fn into_path(self) -> PathBuf {
+        self.path
     }
 }