some more lintcheck changes

[matthiaskrgr]

• Explain why tokei is commented out in the lintcheck sources.
• If we specify a custom sources.toml, don't override the preexisting lintcheck logs, but rather start a new log with the filename depending on the sources.toml name.
• Start adding a readme.md to clippy_dev and add some information on how to use the lintcheck subcommand.
• Add support for path/local sources (I needed this for the next item which is: )
• Collect ICEs that happen while clippy checks crates

changelog: more lintcheck changes

[rust-highfive]

r? @flip1995

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[flip1995]

Just the formatting of the README. Otherwise LGTM

[flip1995]

Suggested change

	## Clippy-dev is a tool to ease clippys development, similar to `rustc`s `x.py`.
	# Clippy Dev Tool
	The Clippy Dev Tool is a tool to ease Clippys development, similar to `rustc`s `x.py`.

[flip1995]

Suggested change

	# lintcheck
	## `lintcheck`

[matthiaskrgr]

@bors r=flip1995

[bors]

📌 Commit a6d493d has been approved by flip1995

[bors]

⌛ Testing commit a6d493d with merge 8dbcffe...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing 8dbcffe to master...