Honor -exclude for calls to same-package functions

[Eljees]

Fixes #251.

Bug

-exclude (checker.Exclusions.Symbols) matches a called function's fully-qualified name, but the name was only ever computed for calls that go through a selector expression (pkg.Func() / recv.Method(), handled in selectorAndFunc). A call to a package-level function from within its own package has no selector — it's a plain identifier (FirstFunc(), not first.FirstFunc()) — so namesForExcludeCheck returned nil for it and the exclusion never matched, even though the same function is correctly excluded when called from an importing package.

Fix

When the selector lookup fails, also try resolving the call as a plain identifier via typesInfo.ObjectOf, and compute FullName() the same way the selector branch does. This is purely additive: the existing selector path is untouched, and non-function identifiers (ok stays false) fall through exactly as before.

Testing

Added TestSamePackageFunctionExclude, modeled on the existing TestTypeParameterizedFunctionExclude: two synthetic packages, one (first) declaring FirstFunc and calling it internally (unqualified) from ThirdFunc, the other (second) calling it externally via first.FirstFunc().

Without the fix, excluding samepkgtest/first.FirstFunc still leaves the in-package call flagged — verified this fails for the expected reason:

errcheck_test.go:373: expected 0 errors (both in-package and cross-package calls excluded), got 1: [{first/first.go:12:11 FirstFunc() }]

With the fix, both calls are excluded and the full package test suite (go test ./...) passes.