fix bug of remote_podspec resource copy

[xuzhongping]

Issue: #3

Description

The reason for this bug is that all copy behaviors assume that the podspec file and related resources exist in the same path, but the remote podspec file does not meet this situation, so any copy behavior will produce errors or invalid results.

Idea

• add a temporary directory to download all files described by remote podspec, the path method used before when accessing resources during the construction process is podspec_dir, I changed it to pod_source_dir,it can map the path according to local or remote.
• All pods will be treated as local pods after the download is complete
• I once thought about whether this problem can be handled in the copy process, but I think it will make the processing of the copy process more complicated, but the current modification puts the processing in the early stage, this will make the subsequent processing flow easier.

[xuzhongping]

@dnkoutso Hi，can you review it?
maybe I should make some changes, I want to hear your opinion.