Support for "watching" directories or files

[ccbrown]

Directory sources should watch their source directory for changes, and be considered out-of-date afterwards.

Other sources might also use additional files or directories as inputs to their build, which should affect them the same way.

For example...

{
    'repository': ...,
    'commit:' ...,
    'project': {
        'post-clean': 'apply-patch {needs_file_directory}/mypatch.diff'
    }
}

For that be appropriately rebuilt, we would need to consider the contents of mypatch.diff. So maybe something like this should be possible:

{
    'repository': ...,
    'commit:' ...,
    'watch': '{needs_file_directory}/mypatch.diff',
    'project': {
        'post-clean': 'apply-patch {needs_file_directory}/mypatch.diff'
    }
}