Add x {check,build,doc} {compiler/library} aliases.

While working on rust-lang#95503,
I realized that this will interfere with existing command lines:
Currently people run `x build library/std` expecting it to be added to the sysroot,
but after that change, it will *only* build `libstd` without making it available
for the toolchain.

It's debatable whether that's a breaking change that will be accepted; if so, this PR is absolutely
necessary to make sure there's a command for "build the standard library and add it to the sysroot".
Even if not, though, I think `x build library` is more clear about what actually happens than the
current `x build library/std`.

For consistency, also add support for `compiler` and all other command variants.  Note that `doc
compiler` was already supported, so in a sense this is just fixing an existing inconsistency.

I plan to change the dev-guide and various instructions in the README to `build library` once this is merged.

src/bootstrap/check.rs

@@ -64,7 +64,7 @@ impl Step for Std {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.all_krates("test")
+        run.all_krates("test").path("library")
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -162,7 +162,7 @@ impl Step for Rustc {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.all_krates("rustc-main")
+        run.all_krates("rustc-main").path("compiler")
     }
 
     fn make_run(run: RunConfig<'_>) {

src/bootstrap/compile.rs

@@ -43,7 +43,7 @@ impl Step for Std {
         // When downloading stage1, the standard library has already been copied to the sysroot, so
         // there's no need to rebuild it.
         let download_rustc = run.builder.config.download_rustc;
-        run.all_krates("test").default_condition(!download_rustc)
+        run.all_krates("test").path("library").default_condition(!download_rustc)
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -1047,7 +1047,7 @@ impl Step for Assemble {
     const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.path("compiler/rustc")
+        run.path("compiler/rustc").path("compiler")
     }
 
     fn make_run(run: RunConfig<'_>) {

src/bootstrap/doc.rs

@@ -417,7 +417,7 @@ impl Step for Std {
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let builder = run.builder;
-        run.all_krates("test").default_condition(builder.config.docs)
+        run.all_krates("test").path("library").default_condition(builder.config.docs)
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -479,11 +479,14 @@ impl Step for Std {
             .iter()
             .map(components_simplified)
             .filter_map(|path| {
-                if path.get(0) == Some(&"library") {
+                if path.len() >= 2 && path.get(0) == Some(&"library") {
+                    // single crate
                     Some(path[1].to_owned())
                 } else if !path.is_empty() {
+                    // ??
                     Some(path[0].to_owned())
                 } else {
+                    // all library crates
                     None
                 }
             })