Don't print absolute paths when:
(1) Printing the "location" of ERROR/INFO/etc messages. Instead, print a path relative to the workspace or a --package_path root. Behavior is controlled by flag --experimental_ui_attempt_to_print_relative_paths).
(2) Printing the absolute path to a test log. Instead print a relative path under the "testlogs" convenience symlink. Behavior is controlled by flag --print_relative_test_log_paths.

There are drawbacks to both these new features (discussed below and also in the flag help strings).

Out of scope (these can be addressed in followup feature requests, as necessary):
(3) Arbitrary strings in messages.
(4) Non-usefulness of relative paths when running Blaze underneath the workspace directory.
(5) Non-usefulness of the a path that contains the "testlogs" convenience symlink when changing configurations in a subsequent invocation.
(6) Other things besides error message prefixes and test log paths in test summaries.

RELNOTES: None
PiperOrigin-RevId: 218762875