install: allow installation of local packages

It may not be desirable to host all plugins on npm, allow for local packages to
be installed given a package name with a `file:` prefix.

This is still more restrictive than what yarn would support but allows us to still
verify the thelounge compatibility by reading the package.json file.

`yarn add` messes up with local filepaths and generates a lockfile that is
"outdated" as far as any other yarn commands go, which makes them error out.

For some reason `yarn install` fixes that and hence we run that after an install.
Here's the diff of yarn.lock between the broken state after `yarn add file:$path`
and `yarn install`
	--- yarn.lock.2.afterAdd	2021-06-02 00:10:52.365134018 +0200
	+++ yarn.lock.3.afterinstall	2021-06-02 00:13:27.122760442 +0200
	@@ -2194,7 +2194,7 @@
	     safe-buffer "^5.1.2"
	     yallist "^3.0.3"

	-thelounge-plugin-shortcuts@/home/reto/sourcecode/thelounge-plugin-shortcuts:
	+"thelounge-plugin-shortcuts@file:../../sourcecode/thelounge-plugin-shortcuts":
	   version "1.0.12"
	   dependencies:
	     thelounge "4.2.0"

The only thing it does is switch an absolute path to a relative one for whatever
reason.

src/command-line/install.js

@@ -13,6 +13,8 @@ program
 	.on("--help", Utils.extraHelp)
 	.action(function (packageName) {
 		const fs = require("fs");
+		const fspromises = fs.promises;
+		const path = require("path");
 		const packageJson = require("package-json");
 
 		if (!fs.existsSync(Helper.getConfigPath())) {
@@ -21,22 +23,31 @@ program
 		}
 
 		log.info("Retrieving information about the package...");
+		let readFile = null;
+		let isLocalFile = false;
 
+		if (packageName.startsWith("file:")) {
+			isLocalFile = true;
+			readFile = fspromises
+				.readFile(path.join(packageName.substr("file:".length), "package.json"), "utf-8")
+				.then((data) => JSON.parse(data));
+		} else {
 			const split = packageName.split("@");
 			packageName = split[0];
 			const packageVersion = split[1] || "latest";
 
-		packageJson(packageName, {
+			readFile = packageJson(packageName, {
 				fullMetadata: true,
 				version: packageVersion,
-		})
+			});
+		}
+
+		readFile
 			.then((json) => {
+				const humanVersion = isLocalFile ? packageName : `${json.name} v${json.version}`;
+
 				if (!("thelounge" in json)) {
-					log.error(
+					log.error(`${colors.red(humanVersion)} does not have The Lounge metadata.`);
-						`${colors.red(
-							json.name + " v" + json.version
-						)} does not have The Lounge metadata.`
-					);
 
 					process.exit(1);
 				}
@@ -47,7 +58,7 @@ program
 				) {
 					log.error(
 						`${colors.red(
-							json.name + " v" + json.version
+							humanVersion
 						)} does not support The Lounge v${Helper.getVersionNumber()}. Supported version(s): ${
 							json.thelounge.supports
 						}`
@@ -56,20 +67,23 @@ program
 					process.exit(2);
 				}
 
-				log.info(`Installing ${colors.green(json.name + " v" + json.version)}...`);
+				log.info(`Installing ${colors.green(humanVersion)}...`);
-
+				const yarnVersion = isLocalFile ? packageName : `${json.name}@${json.version}`;
-				return Utils.executeYarnCommand("add", "--exact", `${json.name}@${json.version}`)
+				return Utils.executeYarnCommand("add", "--exact", yarnVersion)
 					.then(() => {
-						log.info(
+						log.info(`${colors.green(humanVersion)} has been successfully installed.`);
-							`${colors.green(
+
-								json.name + " v" + json.version
+						if (isLocalFile) {
-							)} has been successfully installed.`
+							// yarn v1 is buggy if a local filepath is used and doesn't update
-						);
+							// the lockfile properly. We need to run an install in that case
+							// even though that's supposed to be done by the add subcommand
+							return Utils.executeYarnCommand("install").catch((err) => {
+								throw `Failed to update lockfile after package install ${err}`;
+							});
+						}
 					})
 					.catch((code) => {
-						throw `Failed to install ${colors.green(
+						throw `Failed to install ${colors.red(humanVersion)}. Exit code: ${code}`;
-							json.name + " v" + json.version
-						)}. Exit code: ${code}`;
 					});
 			})
 			.catch((e) => {