vfs: use 'vfs' module directly in 'mercurial.hg' Now that the 'vfs' classes moved in their own module, lets use the new module directly. We update code iteratively to help with possible bisect needs in the future.

vfs: extract 'vfs' class and related code to a new 'vfs' module (API) The 'scmutil' is growing large (1500+ lines) and 2/5 of it is related to vfs. We extract the 'vfs' related code in its own module get both module back to a better scale and clearer contents. We keep all the references available in 'scmutil' for now as many reference needs to be updated.

vfs: replace 'scmutil.opener' usage with 'scmutil.vfs' The 'vfs' class is the first class citizen for years. We remove all usages of the older API. This will let us remove the old API eventually.