Add AllMetadataFiles() method to track deps.

Improve METADATA parsing error message.

Example error message (for an unqoted string url starting with ssh:):

============
  error in project "<redacted>" METADATA "<redacted>/METADATA": proto: (line 4:12): invalid value for string type: ssh

METADATA and METADATA.android files must parse as text protobufs
defined by
   build/soong/compliance/project_metadata_proto/project_metadata.proto

* unknown fields don't matter
* check invalid ENUM names
* check quoting
* check unescaped nested quotes
* check the comment marker for protobuf is '#' not '//'

if importing a library that uses a different sort of METADATA file, add
a METADATA.android file beside it to parse instead
============

Bug: 254534552

Test: m droid dist cts alllicensemetadata

Change-Id: Ie5c0adc7362941e455dd522baaa31f2913d7db5f