ignore: dynamically collect dvcignore (#4284)
ignore: dynamically collect dvcignore (#4284)

This allows us avoid collecting dvcignore for the whole repo if we only
care about particular paths. As a result, in a repo with 2 datasets
(2M + 0.5M files), creating a defunct stage takes ~4sec on 1.2.0, but
~1sec(most of it is actually dvc module initialization) with this PR.

This is also a pre-requisite for dynamic dvcignore and subrepo
collection (https://github.com/iterative/dvc/pull/4247) while walking
the tree.

Also, it is important to clarify that regular `dvc status`(without
arguments) has the same performance after this PR, because when we check
dataset for changes, we call things like `tree.exists()`, which call
dvcignore and make it collect dvcignore in the dataset itself, so we
still endup collecting dvcignore for the whole repo (including walking
into the datasets). This should be solved soon by telling dvcignore that
it shouldn't walk into the datasets searching for `.dvcignore`s.
dmpetrov/dataversioncontrol,efiop/dvc,efiop/dvc,dmpetrov/dataversioncontrol