vet: allow to overwrite excluded dirs

[ttytm]

Allows to run vet over paths that include e.g. tests or testdata when explictly passing them. Currently, they will be excluded without any information.

# Current
v vet vlib/v/fmt/testdata # Silently skipped

# With changes
v vet vlib/v/fmt/testdata # Vetted when explicitly specified
v vet vlib/v/fmt # Subdir /testdata will skipped

This feature also removes the need for keeping a flag that was used for vet development / test purposes.

E.g. checking out the first commit that just removes the flag before extending the functionality and running below will show the problems solved.

v cmd/tools/vvet/vet_test.v

[spytheman]

Excellent work.