platforms: add subarchless version of Only()

`OnlyStrict()` returns a match comparer for a single platform.

Unlike `Only()`, `OnlyStrict()` does not match sub platforms.
So, "arm/vN" will not match "arm/vM" where M < N, and "amd64" will not also match "386".

`OnlyStrict()` matches non-canonical forms. So, "arm64" matches "arm/64/v8".

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

platforms/defaults.go

@@ -36,3 +36,8 @@ func DefaultSpec() specs.Platform {
 		Variant: cpuVariant,
 	}
 }
+
+// DefaultStrict returns strict form of Default.
+func DefaultStrict() MatchComparer {
+	return OnlyStrict(DefaultSpec())
+}