This reverts commit 503ba7c4.

The script keeps state in the objects representing the repository files,
and it also maintains a cache of licenses.  This state should be freshly
recreated for each component so data from previous components does not
leak through.

Add MultiLicense license class

The first 23 lines of the BoringSSL license contain informational text
including Buganizer issue numbers that is not part of the license text
itself.

* Add OpenSSL as supported licence type

Adds special handling for the OpenSSL licensed files under BoringSSL.
Specifically, some of the ARM assembly files in the latest Dart SDK are
dual-licensed with OpenSSL.

* Support blocks that ref by type with no copyright

The upcoming Dart SDK includes files (e.g., sha256-armv4.S) with a block
that specifies teh code is dual-licenced under OpenSSL, but doesn't
include a copyright date of its own.

* Test that licenses are valid.

* Move license script from flutter/buildroot to flutter/engine