fix SOURCE_DATE_EPOCH message spam

[radoering]

The SOURCE_DATA_EPOCH message is printed for each file. Example output of a simple project with almost no files:

$poetrye build
Building foo (0.1.0)
  - Building sdist
SOURCE_DATE_EPOCH environment variable is not set, using mtime=0
SOURCE_DATE_EPOCH environment variable is not set, using mtime=0
SOURCE_DATE_EPOCH environment variable is not set, using mtime=0
SOURCE_DATE_EPOCH environment variable is not set, using mtime=0
  - Built foo-0.1.0.tar.gz
  - Building wheel
SOURCE_DATE_EPOCH environment variable not set, setting zipinfo date to default=(2016, 1, 1, 0, 0, 0)
SOURCE_DATE_EPOCH environment variable not set, setting zipinfo date to default=(2016, 1, 1, 0, 0, 0)
SOURCE_DATE_EPOCH environment variable not set, setting zipinfo date to default=(2016, 1, 1, 0, 0, 0)
SOURCE_DATE_EPOCH environment variable not set, setting zipinfo date to default=(2016, 1, 1, 0, 0, 0)
  - Built foo-0.1.0-py3-none-any.whl

I converted the static methods into cached properties to make sure the messages are only printed once.

Further, I wonder if we should print the message per default at all or only in verbose mode. Therefore, I changed the log level from info to debug.