Automatically determine appropriate rules without the need for `exclude`

[CoolOppo]

Let's say you have the following rules:

[minify]
from = '(?P<name>.*)\.js$'
to = '$name.min.js'
exclude = '\.min\.js$'
command = 'cp $i $o'

[gzip]
from = '(?P<name>.*)\.min\.js$'
to = '$name.min.js.gz'
command = 'zopfli $i'

Without the exclude property, the minify rule will run on its own output files as well. It shouldn't be required in this situation because the program can automatically determine that the .min.js file was generated by that rule, but this situation is not accounted for at the moment.

[CoolOppo]

I'm thinking the way to fix this is to make a struct and store the rule that generated the file along with the file. The rule is also stored in the graph edge pointing to the file as well though, so that might be the real way to go.