Directory restructure (#1532)

Closes #1487
Closes #1427

Directory restructure in accordance with #1487. Also has temporary migration code to move the old directories into new structure.
Also extracts all default directory names and utility functions into a `directory` crate to avoid repetitio.

~Since `validator_definition.yaml` stores absolute paths, users will have to manually change the keystore paths or delete the file to get the validators picked up by the vc.~. `validator_definition.yaml` is migrated as well from the default directories.

Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: Paul Hauner <paul@paulhauner.com>

common/directory/src/lib.rs

@@ -0,0 +1,60 @@
+use clap::ArgMatches;
+pub use eth2_testnet_config::DEFAULT_HARDCODED_TESTNET;
+use std::fs::create_dir_all;
+use std::path::{Path, PathBuf};
+
+/// Names for the default directories.
+pub const DEFAULT_ROOT_DIR: &str = ".lighthouse";
+pub const DEFAULT_BEACON_NODE_DIR: &str = "beacon";
+pub const DEFAULT_NETWORK_DIR: &str = "network";
+pub const DEFAULT_VALIDATOR_DIR: &str = "validators";
+pub const DEFAULT_SECRET_DIR: &str = "secrets";
+pub const DEFAULT_WALLET_DIR: &str = "wallets";
+
+/// Base directory name for unnamed testnets passed through the --testnet-dir flag
+pub const CUSTOM_TESTNET_DIR: &str = "custom";
+
+/// Gets the testnet directory name
+///
+/// Tries to get the name first from the "testnet" flag,
+/// if not present, then checks the "testnet-dir" flag and returns a custom name
+/// If neither flags are present, returns the default hardcoded network name.
+pub fn get_testnet_name(matches: &ArgMatches) -> String {
+    if let Some(testnet_name) = matches.value_of("testnet") {
+        testnet_name.to_string()
+    } else if matches.value_of("testnet-dir").is_some() {
+        CUSTOM_TESTNET_DIR.to_string()
+    } else {
+        eth2_testnet_config::DEFAULT_HARDCODED_TESTNET.to_string()
+    }
+}
+
+/// Checks if a directory exists in the given path and creates a directory if it does not exist.
+pub fn ensure_dir_exists<P: AsRef<Path>>(path: P) -> Result<(), String> {
+    let path = path.as_ref();
+
+    if !path.exists() {
+        create_dir_all(path).map_err(|e| format!("Unable to create {:?}: {:?}", path, e))?;
+    }
+
+    Ok(())
+}
+
+/// If `arg` is in `matches`, parses the value as a path.
+///
+/// Otherwise, attempts to find the default directory for the `testnet` from the `matches`
+/// and appends `flag` to it.
+pub fn parse_path_or_default_with_flag(
+    matches: &ArgMatches,
+    arg: &'static str,
+    flag: &str,
+) -> Result<PathBuf, String> {
+    clap_utils::parse_path_with_default_in_home_dir(
+        matches,
+        arg,
+        PathBuf::new()
+            .join(DEFAULT_ROOT_DIR)
+            .join(get_testnet_name(matches))
+            .join(flag),
+    )
+}