tools/linters: Add copied golangci-lint lll linter code

In this commit, we copy the implementation of the golangci-lint lll
linter which we currently use in our CI flow during the `make lint`
check. This commit copies the code mostly as is (the only exception
being the line which trims white space before checking if a line starts
with "import"), and formats it to fit our codebase guidelines. A test is
also added so that we can be sure that the following commits which
adjust the implementation have the intended results.

The custom linter is put into its own module as this is requied by the
`golangci-lint custom` when building the custom linter binary which
includes the plugin.

tools/linters/go.mod

@@ -0,0 +1,44 @@
+module github.com/lightningnetwork/lnd/tools/linters
+
+go 1.22.6
+
+require (
+	github.com/golangci/golangci-lint v1.62.2
+	github.com/stretchr/testify v1.10.0
+	golang.org/x/tools v0.27.0
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/fatih/color v1.18.0 // indirect
+	github.com/fsnotify/fsnotify v1.5.4 // indirect
+	github.com/go-viper/mapstructure/v2 v2.2.1 // indirect
+	github.com/hashicorp/go-version v1.7.0 // indirect
+	github.com/hashicorp/hcl v1.0.0 // indirect
+	github.com/ldez/gomoddirectives v0.2.4 // indirect
+	github.com/magiconair/properties v1.8.6 // indirect
+	github.com/mattn/go-colorable v0.1.13 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mitchellh/go-homedir v1.1.0 // indirect
+	github.com/mitchellh/mapstructure v1.5.0 // indirect
+	github.com/pelletier/go-toml v1.9.5 // indirect
+	github.com/pelletier/go-toml/v2 v2.2.3 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/rogpeppe/go-internal v1.13.1 // indirect
+	github.com/sirupsen/logrus v1.9.3 // indirect
+	github.com/spf13/afero v1.11.0 // indirect
+	github.com/spf13/cast v1.5.0 // indirect
+	github.com/spf13/jwalterweatherman v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.5 // indirect
+	github.com/spf13/viper v1.12.0 // indirect
+	github.com/stretchr/objx v0.5.2 // indirect
+	github.com/subosito/gotenv v1.4.1 // indirect
+	golang.org/x/exp v0.0.0-20240909161429-701f63a606c0 // indirect
+	golang.org/x/mod v0.22.0 // indirect
+	golang.org/x/sync v0.9.0 // indirect
+	golang.org/x/sys v0.27.0 // indirect
+	golang.org/x/text v0.18.0 // indirect
+	gopkg.in/ini.v1 v1.67.0 // indirect
+	gopkg.in/yaml.v2 v2.4.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)