Update swift package archive-source command to provide directory prefix

[mattt]

Updates swift package archive-source command according to the SE-0292 amendment.

Motivation:

With #3624, the behavior of the archive-source command now adds a --prefix argument to the git-archive invocation, so that entries are prefixed by output name.

Modifications:

  try callGit("archive",
              "--format", "zip",
+             "--prefix", path.basenameWithoutExt,
              "--output", path.pathString,
              "HEAD",
              failureMessage: "Couldn’t create an archive")

Result:

Generated source archives are prefixed by output name.

For example:

$ swift package archive-source --output="LinkedList-1.2.0.zip"
$ zipinfo LinkedList-1.2.0.zip
Archive:  path/to/LinkedList-1.2.0.zip
[...]
[...] LinkedList-1.2.0/
[...] LinkedList-1.2.0/Package.swift

[mattt]

@swift-ci Please smoke test