fix default network handling (#2029)

## Issue Addressed
#1992 and #1987, and also to be considered a continuation of #1751

## Proposed Changes
many changed files but most are renaming to align the code with the semantics of `--network` 
- remove the `--network` default value (in clap) and instead set it after checking the `network` and `testnet-dir` flags
- move `eth2_testnet_config` crate to `eth2_network_config`
- move `Eth2TestnetConfig` to `Eth2NetworkConfig`
- move `DEFAULT_HARDCODED_TESTNET` to `DEFAULT_HARDCODED_NETWORK`
- `beacon_node`s `get_eth2_testnet_config` loads the `DEFAULT_HARDCODED_NETWORK` if there is no network nor testnet provided
- `boot_node`s config loads the config same as the `beacon_node`, it was using the configuration only for preconfigured networks (That code is ~1year old so I asume it was not intended)
- removed a one year old comment stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)
- remove `lighthouse`s `load_testnet_config` in favor of `get_eth2_network_config` to centralize that logic (It had differences)
- some spelling

## Additional Info
Both the command of #1992 and the scripts of #1987 seem to work fine, same as `bn` and `vc`

common/eth2_config/src/lib.rs

@@ -5,13 +5,13 @@ use types::{ChainSpec, EthSpecId};
 
 // A macro is used to define this constant so it can be used with `include_bytes!`.
 #[macro_export]
-macro_rules! testnets_dir {
+macro_rules! predefined_networks_dir {
     () => {
-        "built_in_testnet_configs"
+        "built_in_network_configs"
     };
 }
 
-pub const TESTNETS_DIR: &str = testnets_dir!();
+pub const PREDEFINED_NETWORKS_DIR: &str = predefined_networks_dir!();
 pub const GENESIS_FILE_NAME: &str = "genesis.ssz";
 pub const GENESIS_ZIP_FILE_NAME: &str = "genesis.ssz.zip";
 
@@ -57,7 +57,7 @@ impl Eth2Config {
 
 /// A directory that can be built by downloading files via HTTP.
 ///
-/// Used by the `eth2_testnet_config` crate to initialize testnet directories during build and
+/// Used by the `eth2_network_config` crate to initialize the network directories during build and
 /// access them at runtime.
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub struct Eth2NetArchiveAndDirectory<'a> {
@@ -73,7 +73,7 @@ impl<'a> Eth2NetArchiveAndDirectory<'a> {
             .expect("should know manifest dir")
             .parse::<PathBuf>()
             .expect("should parse manifest dir as path")
-            .join(TESTNETS_DIR)
+            .join(PREDEFINED_NETWORKS_DIR)
             .join(self.unique_id)
     }
 
@@ -94,7 +94,7 @@ macro_rules! define_net {
                 genesis_is_known: $genesis_is_known,
             };
 
-            // A wrapper around `std::include_bytes` which includes a file from a specific testnet
+            // A wrapper around `std::include_bytes` which includes a file from a specific network
             // directory. Used by upstream crates to import files at compile time.
             #[macro_export]
             macro_rules! $macro_title {
@@ -102,7 +102,7 @@ macro_rules! define_net {
                     include_bytes!(concat!(
                         $base_dir,
                         "/",
-                        testnets_dir!(),
+                        predefined_networks_dir!(),
                         "/",
                         $name,
                         "/",