fix: Prevented double -v4 substitution in the update-v4 schematic (20.3.x)

[Ionaru]

Fixes #99

Problem

ng g @angular-architects/native-federation-v4:update-v4 <project> rewrites the federation config import to @angular-architects/native-federation-v4-v4/config (doubled -v4), which does not resolve, so the build fails until corrected by hand.

Cause

Two substitution passes in migrateFederationConfigs compound:

1. The require-to-import rewrite already updates the module path to @angular-architects/native-federation-v4/config.
2. The bare-package replacement then matches the v3 prefix inside that already-substituted specifier, because its (?!/) lookahead only guards against a following / and the next character here is -. It appends a second -v4.

Note: reordering the replacements (as suggested as an alternative in the issue) would not help, since the first substitution happens in the require-to-import rewrite before either content-level replacement runs.

Fix

Guard both substitutions against an already-substituted prefix:

• The require-path rewrite now uses a (?!-v4) lookahead.
• The bare-package replacement lookahead is widened from (?!/) to (?!/|-v4).

The conservative semantics of the bare replacement (skip subpaths other than /config) are unchanged.

Tests

Added packages/angular/src/schematics/update-v4/schematic.spec.ts covering the reported repro (CJS config on v3) and idempotence (a require path already on v4 stays single -v4). Both failed before the fix, reproducing -v4-v4/config and -v4-v4-v4/config respectively, and pass after. Full suite, typecheck, and lint are clean.

This is the same change as #102, ported to the 20.3.x branch (monorepo layout).