The "all:" prefix in //go:embed directives instructs the compiler to
include hidden files (starting with "." or "_") when embedding a
directory. Previously, the prefix was passed literally to path.Match,
which would never match.
Introduce an embedPattern struct that parses and validates the pattern
at construction time via newEmbedPattern(). The "all:" prefix is
stripped once and stored along with the glob pattern, avoiding repeated
string prefix checks. Pattern validation is absorbed into the
constructor, guaranteeing that any embedPattern value is valid and
eliminating the need for separate validation loops.
Fixes embedding with patterns like "//go:embed all:static".
Abhishek Goyal
Ayke van Laethem