Adds a Fingerprinter utility class that can be used to compute unique
fingerprints for a set of input paths and build options, compare to the
output of a previous run, and skip the build action if no inputs or
options have changed. The existing Fingerprint class still does all the
heavy lifting. Fingerprinter adds common operations such as
reading/writing/comparing fingerprints and parsing depfiles.

This migrates existing uses of Fingerprint over to Fingerprinter.

This also adds better fingerprinting to AOT snapshotting, which
previously failed to include several options in its fingerprint
(--preview-dart-2, --prefer-shared-library).