refactor: move parse and related function to common

[furtib]

Why:
We want to have feature and behaviour parity between the monolithic and the per-file rule. To achieve this, we want these modules to share code where it's possible.

Related: #315

What:

• Moved code related to parsing and severity checking to common.py.

Addresses:
none

Depends on: #342

[furtib]

CI fails on unchanged files.
The reason is a module name collision between the common in test/common and the common in src/common.

Since we plan on removing test/common after merging all pytest-to-Bazel PRs (#329 #298 #318), I think we should merge those and, if necessary, change the name of test/common, not this one.

[Szelethus]

I figured a primary motivation behind this patch was unifying per_file and monolithic runs, but I see that you don't call parse at all from per_file. Is this intentional?

[furtib]

The primary motivation in this PR was to split the #315 patch into two.
One where I don't change the behaviour of the per-file rule, only refactor (this PR), and another (this will be #315) where I do change the behaviour of the per-file rule.
The per-file rule so far only performs the analysis step, which is not suitable for unification.
Since the per-file rule currently does not perform a parsing step, I figured a non-functional change should not introduce that either.

[Szelethus]

I caught a few things from #345 that would be better placed here. In particular, what about src/per_file.bzl:117, which is also adds "--mode=Run" to the invocation of the per-file script?

[Szelethus]

LGTM; I'm sure we'll catch something that could've been moved here, but at that point we are making our life harder for no gain. We want this patch, so lets not delay it any longer.