Relocate the top-level validate() into the hdmf.validate package

[rly]

Motivation

Part of the #608 cleanup. The public top-level validate() currently lives in src/hdmf/common/__init__.py, but hdmf.common is the module for the hdmf-common data types; validation is orthogonal to it. The hdmf.validate subpackage already houses the validator classes (ValidatorMap, Validator, ...) but exposes no validate() function.

Proposed change (target a major release)

• Define validate(io, ...) in the hdmf.validate subpackage and export it from src/hdmf/validate/__init__.py. With the namespace-aware dispatcher (Validate against all loaded namespaces via a namespace-aware dispatcher (fixes #608) #1529), the body is a single validation pass over the whole file's builder tree against all loaded namespaces. No per-namespace scoping argument (per-namespace scoping of a whole-file validation is exactly the Account for all loaded namespaces during validation #608 bug).
• Keep hdmf.common.validate as a thin back-compat shim delegating to hdmf.validate.validate; the namespace/experimental scoping args are accepted-but-ignored with a deprecation note.

Depends on #1529 (the namespace-aware validation dispatcher).

Acceptance criteria

• hdmf.validate.validate(io) validates against all loaded namespaces in one pass.
• hdmf.common.validate(...) still imports and works (deprecated shim).
• Docs/CHANGELOG updated for the new canonical location and the deprecation.