Give consistent sources to each plugin when migrating

Currently, the list of sources in a project is recalculated after each
plugin is run. Some plugins, like "explicitAny" and
"declareMissingClasProperties", expand the scope of a project by pulling
dependencies of the file they're modifying into the project. This means
that the order in which plugins run impacts the files that ts-migrate
touches.

This commit finds sources for a pronect once, before any plugins are
run, and re-uses them for each plugin. This prevents files from being
pulled in, which is particularly surprising if the '--sources' flag is
in use.

packages/ts-migrate-server/src/migrate/index.ts

@@ -50,6 +50,9 @@ export default async function migrate({
   log.info('Start...');
   const pluginsTimer = new PerfTimer();
   const updatedSourceFiles = new Set<string>();
+  const sourceFiles = project
+    .getSourceFiles()
+    .filter(({ fileName }) => !/(\.d\.ts|\.json)$|node_modules/.test(fileName));
 
   for (let i = 0; i < config.plugins.length; i += 1) {
     const { plugin, options: pluginOptions } = config.plugins[i];
@@ -58,10 +61,6 @@ export default async function migrate({
     const pluginTimer = new PerfTimer();
     log.info(`${pluginLogPrefix} Plugin ${i + 1} of ${config.plugins.length}. Start...`);
 
-    const sourceFiles = project
-      .getSourceFiles()
-      .filter(({ fileName }) => !/(\.d\.ts|\.json)$|node_modules/.test(fileName));
-
     // eslint-disable-next-line no-restricted-syntax
     for (const sourceFile of sourceFiles) {
       const { fileName } = sourceFile;