Currently the double-star pattern term `**` behaves like a star `*`,
as one star matches any sequence of non-separator characters
and the second star matches nothing.

This is confusing, as one may write the pattern `**/*.json` and it can
match strings like `foo/bar.json`, making it seem like `**` works as
a double star that would also match `foo.json` or `foo/bar/baz.json`
when in fact it does not.

If we choose to support `**` patterns in the future to match separators,
this would mean changing the behavior for existing users in a subtle way
which could lead to unintended behavior or latent bugs.
It is clearer for existing users to treat `**` as an invalid pattern,
which would allow us to add the feature in the future without concern
about existing users already using such patterns without error.

We also mirror Go's path.Match behavior to consume the entire pattern
to validate that it is always syntactically valid, so that we may
use it to check for `**` even when the matching has otherwise ended.
See https://go.dev/issue/28614 for details; we largely copied these
semantics from Go 1.15 already, but we omitted a bit of code that was
necessary for trailing double-stars to be caught as an error.

This change is driven by the embed proposal, where recursively embedding
entire directory trees is a relatively common use case that we want
to support somehow, and users might be misled by `**` silently working
in a way that doesn't match the user's expectation. We want path.Match,
tool/file.Glob, and embed patterns to behave consistently, so for that
reason, start consistently treating `**` as an error.

Note that this is a breaking change for any users whose pattern
includes `**` as two consecutive wildcards, and this is on purpose
for the reasons outlined above. Users who run into this problem
may not have been aware that it behaved like a single wildcard,
and in the majority of cases they should be able to use `*` instead.

For #1919.
Fixes #3315.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ifeb81f4818a863cd0e4c5c13bc0fc2ac54a0f22f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198636
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>