refactor(readTree): Move 'restrictFolder' function into readTree r/3082

This is generally useful for readTree users and should be part of
readTree itself.

This is a move towards exposing several readTree-related features from
the library itself, in the future also including logic like 'gather'.

Note that this has a small functional change: In error messages of the
function, the notation for accessing Nix attributes is now used rather
than the Perforce-style `//` notation common in TVL.

For example, an error at `//web/tvl/logo` will produce `web.tvl.logo`
in the error message (which corresponds to the readTree attribute
itself).

This makes more sense for non-TVL consumers of readTree, as the
Perforce-style notation is custom to us specifically.

Change-Id: I8e199e473843c40db40b404c20d2c71f48a0f658

0 files changed, 0 insertions, 0 deletions