[Workspace] Allow SWIFTPM_PD_LIBS to accept a list of search paths

<rdar://problem/45473237> SWIFTPM_PD_LIBS should accept a list of paths

Author: aciidgh

Documentation/Development.md

@@ -183,4 +183,7 @@ $ SWIFT_EXEC=/path/to/my/built/swiftc swift build
 
 SwiftPM computes the path of runtime libraries relative to where it is
 installed. This path can be overridden by setting the environment variable
-`SWIFTPM_PD_LIBS` to a directory containing the libraries.
+`SWIFTPM_PD_LIBS` to a directory containing the libraries. This can be a list of
+absolute search paths separated by colon (":"). SwiftPM will choose the first
+path which exists on disk. If none of the path are present on disk, it will fall
+back to built-in computation.

Sources/Workspace/UserToolchain.swift

@@ -240,11 +240,18 @@ public final class UserToolchain: Toolchain {
         ] + destination.extraCCFlags
 
         // Compute the path of directory containing the PackageDescription libraries.
-        let pdLibDir: AbsolutePath 
-        if let pdLibDirEnvStr = getenv("SWIFTPM_PD_LIBS"), let pdLibDirEnv = try? AbsolutePath(validating: pdLibDirEnvStr) {
-            pdLibDir = pdLibDirEnv
-        } else {
-            pdLibDir = binDir.parentDirectory.appending(components: "lib", "swift", "pm")
+        var pdLibDir = binDir.parentDirectory.appending(components: "lib", "swift", "pm")
+
+        // Look for an override in the env.
+        if let pdLibDirEnvStr = getenv("SWIFTPM_PD_LIBS") {
+            // We pick the first path which exists in a colon seperated list.
+            let paths = pdLibDirEnvStr.split(separator: ":").map(String.init)
+            for pathString in paths {
+                if let path = try? AbsolutePath(validating: pathString), localFileSystem.exists(path) {
+                    pdLibDir = path
+                    break
+                }
+            }
         }
 
         manifestResources = UserManifestResources(