Introduce atom.directory-provider service.

A `Project` will always have a `DefaultDirectoryProvider` that
will be used if there are no other `DirectoryProvider` objects
that can produce a `Directory` for a path.

spec/default-directory-provider-spec.coffee

@@ -0,0 +1,21 @@
+DefaultDirectoryProvider = require "../src/default-directory-provider"
+path = require "path"
+temp = require "temp"
+
+describe "DefaultDirectoryProvider", ->
+  describe ".directoryForURISync(uri)", ->
+    it "returns a Directory with a path that matches the uri", ->
+      provider = new DefaultDirectoryProvider()
+      tmp = temp.mkdirSync()
+
+      directory = provider.directoryForURISync(tmp)
+      expect(directory.getPath()).toEqual tmp
+
+  describe ".directoryForURI(uri)", ->
+    it "returns a Promise that resolves to a Directory with a path that matches the uri", ->
+      provider = new DefaultDirectoryProvider()
+      tmp = temp.mkdirSync()
+
+      waitsForPromise ->
+        provider.directoryForURI(tmp).then (directory) ->
+          expect(directory.getPath()).toEqual tmp

src/default-directory-provider.coffee

@@ -0,0 +1,33 @@
+{Directory} = require 'pathwatcher'
+fs = require 'fs-plus'
+path = require 'path'
+
+module.exports =
+class DefaultDirectoryProvider
+
+  # Public: Create a Directory that corresponds to the specified URI.
+  #
+  # * `uri` {String} The path to the directory to add. This is guaranteed not to
+  #     be contained by a {Directory} in `atom.project`.
+  # Returns:
+  # * {Directory} if the given URI is compatible with this provider.
+  # * `null` if the given URI is not compatibile with this provider.
+  directoryForURISync: (uri) ->
+    projectPath = path.normalize(uri)
+
+    directoryPath = if fs.isDirectorySync(projectPath)
+      projectPath
+    else
+      path.dirname(projectPath)
+
+    new Directory(directoryPath)
+
+  # Public: Create a Directory that corresponds to the specified URI.
+  #
+  # * `uri` {String} The path to the directory to add. This is guaranteed not to
+  #     be contained by a {Directory} in `atom.project`.
+  # Returns a Promise that resolves to:
+  # * {Directory} if the given URI is compatible with this provider.
+  # * `null` if the given URI is not compatibile with this provider.
+  directoryForURI: (uri) ->
+    Promise.resolve(@directoryForURISync(uri))

src/project.coffee

@@ -8,9 +8,9 @@ Q = require 'q'
 {Model} = require 'theorist'
 {Subscriber} = require 'emissary'
 {Emitter} = require 'event-kit'
+DefaultDirectoryProvider = require './default-directory-provider'
 Serializable = require 'serializable'
 TextBuffer = require 'text-buffer'
-{Directory} = require 'pathwatcher'
 Grim = require 'grim'
 
 TextEditor = require './text-editor'
@@ -41,6 +41,14 @@ class Project extends Model
     @rootDirectories = []
     @repositories = []
 
+    @directoryProviders = [new DefaultDirectoryProvider()]
+    atom.packages.serviceHub.consume(
+      'atom.directory-provider',
+      '^0.1.0',
+      # New providers are added to the front of @directoryProviders because
+      # DefaultDirectoryProvider is a catch-all that will always provide a Directory.
+      (provider) => @directoryProviders.unshift(provider))
+
     # Mapping from the real path of a {Directory} to a {Promise} that resolves
     # to either a {Repository} or null. Ideally, the {Directory} would be used
     # as the key; however, there can be multiple {Directory} objects created for
@@ -182,22 +190,22 @@ class Project extends Model
     Grim.deprecate("Use ::setPaths instead")
     @setPaths([path])
 
-  # Public: Add a path the project's list of root paths
+  # Public: Add a path to the project's list of root paths
   #
   # * `projectPath` {String} The path to the directory to add.
   addPath: (projectPath, options) ->
-    projectPath = path.normalize(projectPath)
+    for directory in @getDirectories()
+      # Apparently a Directory does not believe it can contain itself, so we
+      # must also check whether the paths match.
+      return if directory.contains(projectPath) or directory.getPath() is projectPath
 
-    directoryPath = if fs.isDirectorySync(projectPath)
-      projectPath
-    else
-      path.dirname(projectPath)
-
-    return if @getPaths().some (existingPath) ->
-      (directoryPath is existingPath) or
-      (directoryPath.indexOf(path.join(existingPath, path.sep)) is 0)
-
-    directory = new Directory(directoryPath)
+    directory = null
+    for provider in @directoryProviders
+      break if directory = provider.directoryForURISync?(projectPath)
+    if directory is null
+      # This should never happen because DefaultDirectoryProvider should always
+      # return a Directory.
+      throw new Error(projectPath + ' could not be resolved to a directory')
     @rootDirectories.push(directory)
 
     repo = null