[SE-0201] Package Manager Local Dependencies (#816)

* Add local dependencies draft

* Update SE number

Author: neonichu

proposals/0201-package-manager-local-dependencies.md

@@ -0,0 +1,72 @@
+# Package Manager Local Dependencies
+
+* Proposal: [SE-0201](0201-package-manager-local-dependencies.md)
+* Author: [Ankit Aggarwal](https://github.com/aciidb0mb3r)
+* Review Manager: [Boris Bügling](https://github.com/neonichu)
+* Status: **Active Review (March 22...29, 2018)**
+
+## Introduction
+
+This proposal adds a new API in `PackageDescription` to support declaring
+dependency on a package using its path on disk instead of the git URL.
+
+## Motivation
+
+There are two primary motivations for this proposal:
+
+1. Reduce friction in bringing up a set of packages.
+
+	Currently, there is a lot of friction in setting up a new set of interconnected
+packages. The packages must be in a git repository and the package author needs
+to run the `swift package edit` command for each dependency to develop them in tandem.
+
+2. Some package authors prefer to keep their packages in a single repository.
+   This could be because of several reasons, for e.g.:
+
+    * The packages are in very early design phase and are not ready to be published
+    in their own repository yet.
+    * The author has a set of loosely related packages which they may or may not
+    want to publish as a separate repository in future.
+
+## Proposed solution
+
+We propose to add the following API in the `PackageDescription` module:
+
+```swift
+extension Package.Dependency {
+    public static func package(path: String) -> Package.Dependency
+}
+```
+
+* This API will only be available if the tools version of the manifest is
+  greater than or equal to the Swift version this proposal is implemented in.
+
+* The value of `path` must be a valid absolute or relative path string.
+
+* The package at `path` must be a valid Swift package. 
+
+* The local package will be used as-is and the package manager will not try to
+  perform any git operation on the package.
+
+* The local package must be declared in the root package or in other local
+  dependencies. This means, it is not possible to depend on a regular versioned
+  dependency that declares a local package dependency.
+
+* A local package dependency will override any regular dependency in the package
+  graph that has the same package name.
+
+* A local dependency will not be recorded in the `Package.resolved` file and
+  if it overrides a dependency in the package graph, any existing entry will be
+  removed. This is similar to the edit mode behaviour.
+
+* The package manager will not allow using the edit feature on local dependencies.
+
+## Impact on existing packages
+
+None.
+
+## Alternatives considered
+
+We considered piggybacking this on the multi-package repository feature, which
+would also allow authors to publish subpackages from a repository. However, we
+think local dependencies is a much simpler feature that stands on its own.