Refactor nolintlint handling: Enforce directive and linter list formatting

[796RCP92VZ]

The rationale for this PR is to enforce the formatting of the spacing in the nolint directive via the nolintlint linter. The syntax of a Go directive should roughly conform to the following format: ^[a-z]+:[a-z]+. For nolint directive comments that have leading whitespace, the nolintlint linter currently gives a warning of the form “directive // nolint: lll // ignore line length warnings should be written without leading space as //nolint: lll // ignore line length warnings", when it should actually suggest //nolint:lll // ignore line length warnings (with no space between nolint: and lll) instead. Broadly, this PR makes the nolintlint warnings more strictly conform to the syntax of a Go directive and avoids providing inaccurate suggestions such as the aforementioned. This should reduce confusion for developers and should make it easier to conform to the requirements of go fmt as of go1.19.

Change list:

• Remove directiveWithOptionalLeadingSpace field - leading spaces in the directive are no longer allowed as of go1.19, so the directive will now always be //nolint. It cannot be any directive other than nolint (e.g., go:generate) due to the guarantee provided by matching the comment against the commentPattern regular expression in the Run function. Therefore, we can just hardcode //nolint as the directive. (The directive, as the term is generally used in Go, could be something like //nolint:lll or //nolint:all with a colon and specific linter(s) after nolint, but the directiveWithOptionalLeadingSpace variable would not currently include the optional colon and anything following it, so for the purposes of this field, the directive will always be //nolint).
• Update the fullDirectivePattern to disallow spaces in the linter list. The updated regular expression also disallows spaces at the beginning of the string in order to avoid providing an error message that could potentially contain an inaccurate suggestion for how to update the nolint directive, as would have previously been done by the NotMachine issue. The regular expression continues to allow empty explanation comments because they will be caught by the NoExplanation issue if the NeedsExplanation flag is set.
• Unexport BaseIssue struct, since it should not be used outside nolintlint.go.
• Deprecate NeedsMachineOnly and remove all references in code.
• Remove the NotMachine issue, since it has been superseded by the ParserError, which gives a more accurate error message (specifically, it does not provide a suggestion for how to update the nolint directive that could potentially still have spaces in the linter list, but rather a more generic suggestion of the format of a well-formed nolint comment).
• Update NoExplanation details to provide a generic directive template rather than a specific suggestion; update unit tests accordingly. Similar to the previous bullet point, this avoids inadvertently providing an inaccurate error message.
• Remove unused //nolint:gocritic comments.
• Update test cases in the unit test file that should be marked as invalid due to whitespace before/between/after the list of linters.
• Update test cases that previously resulted in the NotMachine issue to reflect that they now result in the ParseError issue instead.
• Add test cases that would have previously erroneously resulted in directiveWithOptionalLeadingSpace values not equal to //nolint or // nolint (namely, directiveWithOptionalLeadingSpace values containing additional characters after the directive, such as //nolint / description).

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