resource: support relative paths as targets of #stage

`#stage` takes an optional `target` argument, which specifies the
location in which to unpack the `resource`.

Passing relative paths to `#stage`, as in

    resource("foo").stage "bar"

does not work, because this calls `Pathname("bar").install` inside the
temporary directory that the contents of the resource are unpacked to.
This means that passing a relative path to `#stage` has the unintended
effect of moving files around inside a temporary directory the formula
has no access to.

Let's fix that by keeping track of the original working directory before
`#stage` was called and prepending this to `target` whenever `target` is
a relative path.

Library/Homebrew/resource.rb

@@ -121,6 +121,7 @@ class Resource
   # is a {ResourceStageContext}.
   # A target or a block must be given, but not both.
   def unpack(target = nil)
+    current_working_directory = Pathname.pwd
     mktemp(download_name) do |staging|
       downloader.stage do
         @source_modified_time = downloader.source_modified_time
@@ -129,6 +130,7 @@ class Resource
           yield ResourceStageContext.new(self, staging)
         elsif target
           target = Pathname(target)
+          target = current_working_directory/target if target.relative?
           target.install Pathname.pwd.children
         end
       end