internal/devconfig: move project directory search into devbox.Find (#2172)

Some cleanup when I looked into supporting different config extensions
(e.g., `devbox.jsonc` to make editors okay with comments).

Instead of first searching for a project directory and then calling
`devbox.Open` on that path, add a `devbox.Find` method that performs the
search and open as a single step.

- Add separate `Find` and `Open` functions to make it clearer when we're
recursively searching for a config vs. not.
- Try just reading files instead of performing a separate `os.Stat`.
- Fix a bug where specifying any filename other than `devbox.json` with
the `-C` flag would fail with a bad error message.

Changes to user error messages:

```diff
 $ devbox add go

-Error: No devbox.json found in this directory, or any parent directories. Did you run `devbox init` yet?
+Error: no devbox.json found in the current directory (or any parent directories). Did you run `devbox init` yet?

 $ devbox -c badpath add go
-Error: stat /var/folders/79/1yc1ywp10w9f2xnr_rpp_ff00000gn/T/tmp.bU25JVWovO/badpath: no such file or directory
+
+Error: the devbox config path "badpath" does not exist.

 $ mkdir child
 $ devbox -c child add go

-Error: No devbox.json found in child. Did you run `devbox init` yet?
+Error: no devbox.json found in "child". Did you run `devbox init` yet?
```

Author: gcurtis

.golangci.yml

@@ -38,6 +38,8 @@ linters-settings:
       - name: bare-return
       - name: bool-literal-in-expr
       - name: cognitive-complexity
+        exclude:
+          - "**_test.go"
         arguments:
           - 32 # TODO: gradually reduce it
       - name: datarace

internal/devbox/devbox.go

@@ -76,14 +76,24 @@ func InitConfig(dir string) (bool, error) {
 }
 
 func Open(opts *devopt.Opts) (*Devbox, error) {
-	projectDir, err := findProjectDir(opts.Dir)
-	if err != nil {
-		return nil, err
+	var cfg *devconfig.Config
+	var err error
+	if opts.Dir == "" {
+		cfg, err = devconfig.Find(".")
+		if errors.Is(err, devconfig.ErrNotFound) {
+			return nil, usererr.New("no devbox.json found in the current directory (or any parent directories). Did you run `devbox init` yet?")
+		}
+	} else {
+		cfg, err = devconfig.Open(opts.Dir)
+		if errors.Is(err, os.ErrNotExist) {
+			return nil, usererr.New("the devbox config path %q does not exist.", opts.Dir)
+		}
+		if errors.Is(err, devconfig.ErrNotFound) {
+			return nil, usererr.New("no devbox.json found in %q. Did you run `devbox init` yet?", opts.Dir)
+		}
 	}
-
-	cfg, err := devconfig.Open(projectDir)
 	if err != nil {
-		return nil, errors.WithStack(err)
+		return nil, usererr.WithUserMessage(err, "Error loading devbox.json.")
 	}
 
 	environment, err := validateEnvironment(opts.Environment)
@@ -96,7 +106,7 @@ func Open(opts *devopt.Opts) (*Devbox, error) {
 		env:                      opts.Env,
 		environment:              environment,
 		nix:                      &nix.Nix{},
-		projectDir:               projectDir,
+		projectDir:               filepath.Dir(cfg.Root.AbsRootPath),
 		pluginManager:            plugin.NewManager(),
 		stderr:                   opts.Stderr,
 		customProcessComposeFile: opts.CustomProcessComposeFile,