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Currently, a CIPD staging directory is populated by copying specified
`sources` into it, and producing a .yaml manifest which reflects the
copied files.

This change refactors the template to invoke a pytho script to do the
above instead of in GN, using the `yaml` library to write the file data
in the proper format.

Also make the license filepath explicitly passed through sources.

Renames "archive" -> "package" and "manifest" -> "package definition" to
be consistent with CIPD semantics (crbug.com/1042819#c8).

Finally, have the script emit a depfile for all files tracked by the
.yaml manifest.

This refactor is desirable, as we want to create CIPD packages with more
complicated rules than simply "copy and include these files" (see
crrev.com/c/1925446, which extends the functionality of the template to
allow for the inclusion of directories). Wriiting yamls with a library
reduces the error-proneness of trying to recreate yaml's format in pure
GN.

Also, emitting a depfile ensure that we track and rebuild if the CIPD
package is modified.

Currently the potential danger of this change is limited in scope to
fuchsia, as they are the only users of the template. I've checked
locally that the produced CIPD packages for fuchsia are identical prior
to this change.

Bug: fuchsia:41443
Change-Id: If1b27d6a8fbbffdf767aaa4230bf7b527a553170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135186
Commit-Queue: Anthony Fandrianto <atyfto@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758433}