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.