ast: report violations from multiple compiler stages

[sspaink]

Compilation stopped at the first stage that recorded an error, so a policy with several violations only ever surfaced one. Errors from checks that leave the modules intact can now be marked as recoverable, so later stages still run, and the collected errors are sorted by location.

The only errors that have been marked recoverable are from stages that are read only and won't have the cascading error issue @johanfylling pointed out.

Message	Stage
import data.foo unused	CheckUnusedImports (new stage)
import must not shadow import data.foo	CheckDuplicateImports
rules must not shadow input|data (use a different rule name)	CheckKeywordOverrides
variables must not shadow input|data (use a different variable name)	CheckKeywordOverrides
args must not shadow input|data (use a different variable name)	CheckKeywordOverrides
unused argument x. (hint: use _ (wildcard variable) instead)	RewriteLocalVars
assigned var x unused	RewriteLocalVars
declared var x unused	RewriteLocalVars

The policy from the issue will now print out 3 errors:

package p

import data.foo

a := any([foo])
b := all([true])

f(x) {
    input.foo
}

BEFORE  1 error occurred: p.rego:8: rego_compile_error: unused argument x. (hint: use _ ...)

AFTER   3 errors occurred:
        p.rego:5: rego_type_error: deprecated built-in function calls in expression: any
        p.rego:6: rego_type_error: deprecated built-in function calls in expression: all
        p.rego:8: rego_compile_error: unused argument x. (hint: use _ ...)

Fixes: #5815