Reorganize top-level config keys

[dcastro]

Clarification and motivation

At the moment, the config yaml is organized in 3 top-level keys: traversal, verification and scanners.

xrefcheck/src/Xrefcheck/Config/Default.hs

Lines 17 to 70 in 31777f7

	traversal:
	# Glob patterns describing files which we pretend do not exist
	# (so they are neither analyzed nor can be referenced).
	ignored:
	# Git files
	- .git/**/*
	# Stack files
	- .stack-work/**/*
	# Verification parameters.
	verification:
	# On 'anchor not found' error, how much similar anchors should be displayed as
	# hint. Number should be between 0 and 1, larger value means stricter filter.
	anchorSimilarityThreshold: 0.5
	# When checking external references, how long to wait on request before
	# declaring "Response timeout".
	externalRefCheckTimeout: 10s
	# Glob patterns describing the files, references in which should not be analyzed.
	notScanned:
	- :PLACEHOLDER:notScanned:
	# Glob patterns describing the files which do not physically exist in the
	# repository but should be treated as existing nevertheless.
	virtualFiles:
	- :PLACEHOLDER:virtualFiles:
	# POSIX extended regular expressions that match external references
	# that have to be ignored (not verified).
	ignoreRefs:
	# Ignore localhost links by default
	- ^(https?|ftps?)://(localhost|127\.0\.0\.1).*
	# Skip links which return 403 or 401 code.
	ignoreAuthFailures: true
	# When a verification result is a "429 Too Many Requests" response
	# and it does not contain a "Retry-After" header,
	# wait this amount of time before attempting to verify the link again.
	defaultRetryAfter: 30s
	# How many attempts to retry an external link after getting
	# a "429 Too Many Requests" response.
	maxRetries: 3
	# Parameters of scanners for various file types.
	scanners:
	markdown:
	# Flavor of markdown, e.g. GitHub-flavor.
	#
	# This affects which anchors are generated for headers.
	flavor: :PLACEHOLDER:flavor:

However, as was discussed in this thread, the distinction between the "traversal" and the "verification" stages is not relevant to the user. This is entirely an internal concern.

A better way of organizing these options would be:

1. exclusions: ignored, notScanned, ...
2. networking: externalRefCheckTimeout, ignoreAuthFailures, defaultRetryAfter, ...
3. scanners

Acceptance criteria

The yaml config options are organized under exclusions, networking and scanners.