Respect direct URLs in puffin installer (#345)

We now write the `direct_url.json` when installing, and _skip_
installing if we find a package installed via the direct URL that the
user is requesting.

A lot of TODOs, especially around cleaning up the `Source` abstraction
and its relationship to `DirectUrl`. I'm gonna keep working on these
today, but this works and makes the requirements clear.

Closes #332.

crates/puffin-cli/tests/snapshots/pip_sync__install_git_commit.snap

@@ -0,0 +1,24 @@
+---
+source: crates/puffin-cli/tests/pip_sync.rs
+info:
+  program: puffin
+  args:
+    - pip-sync
+    - requirements.txt
+    - "--cache-dir"
+    - /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpMEVMlO
+  env:
+    VIRTUAL_ENV: /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpevmjat/.venv
+---
+success: true
+exit_code: 0
+----- stdout -----
+
+----- stderr -----
+Resolved 1 package in [TIME]
+Downloaded 1 package in [TIME]
+Built 1 package in [TIME]
+Unzipped 1 package in [TIME]
+Installed 1 package in [TIME]
+ + werkzeug @ git+https://github.com/pallets/werkzeug.git@af160e0b6b7ddd81c22f1652c728ff5ac72d5c74
+