Reuse existing build phases in CocoaPods plugin

[mihai8804858]

I've encountered a couple of issues when trying to integrate remote cache using CocoaPods plugin, so this is an attempt to fix them. Here is what has been done and the reasoning:

• Handle missing ~/.lldbinit file (our CI nodes doesn't contain this file and pod install fails when trying to read it)
• Reuse exiting prebuild, postbuild and mark build phases instead of everytime deleting and creating new build phases. When creating a new build phase a random id is associated to it which modifies the *.pbxproj file. Because the build phases are deleted / created on each pod install, this also modifies the *.pbxproj. This is very annoying because each time I checkout to a branch / commit I will have unwanted changes to the project. As a workaround, first I check if the build phase already exist and reuse that one , if not then I create a new build phase.
• Use $SRCROOT env variable in build phases instead of absolute paths. My understanding is that these build phases will be committed to the remote, which makes it impossible because the build phases uses absolute paths which are not portable from one machine to another. Using $SRCROOT should make it work for both use cases.

[mihai8804858]

To verify that we haven't leaked any absolute paths, you can take a look on all json file uploaded to the remote server in a meta directory - should be correctly mapped.

I looked at the json files uploaded and can confirm that we didn't leak any absolute paths. I see that there are some absolute paths for the modulemaps in the jsons, but this is also true I'm using the plugin from master branch.

[polac24]

I see that there are some absolute paths for the modulemaps in the jsons

Ups, at glance it sounds like a bug. The json file should not have any paths starting with /. Does your project meet the requirement from the Limitation section?

All compilation files should be referenced via the git repo root. 
Referencing /AbsolutePath/someOther.swift or ../../someOther.swift 
that resolve to the location outside of the git repo root is prohibited.

[mihai8804858]

Thanks @polac24 for quick feedback! <3