feat(spm): add support for local SPM packages

[rigor789]

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• Tests for the changes are included.

What is the current behavior?

There's no way to add local SPM packages.

What is the new behavior?

Local SPM packages can be added using the new package definition:

SPMPacakges: [
  {
    name: 'LocalPackage',
    libs: ['LocalPackageLib'],
    path: './path/to/package' // can be relative to the config or absolute
  },
  {
    name: 'RemotePackage',
    libs: ['RemotePackageLib'],
    repositoryURL: 'remote repository url',
    version: '1.0.0'
  } 
]

Handling implemented in Trapeze PR: ionic-team/trapeze#176 (comment)