Fix component remove error by component name with explicit target triple

[ongchi]

Fix #3166

Follow by #3467, another approach to fix the error.

Rationale

The basic point of this PR is to improve the error message when Rustup somehow cannot recognize a component's full name by extracting the component name first from the manifest when parsing rust-std-x-y-z, so that at least x-y-z is intended to be a target triple. This prevents Rustup from falling back on the default target triple, producing an error message that is not very helpful (see the quote below).

The direct cause of #3166 is that wasm32-unknown-unknown hasn't been hardcoded into our codebase:

rustup/src/dist/triple.rs

Lines 7 to 37 in a6c9fae

	static LIST_ARCHS: &[&str] = &[
	"i386",
	"i586",
	"i686",
	"x86_64",
	"arm",
	"armv7",
	"armv7s",
	"aarch64",
	"mips",
	"mipsel",
	"mips64",
	"mips64el",
	"powerpc",
	"powerpc64",
	"powerpc64le",
	"riscv64gc",
	"s390x",
	"loongarch64",
	];
	static LIST_OSES: &[&str] = &[
	"pc-windows",
	"unknown-linux",
	"apple-darwin",
	"unknown-netbsd",
	"apple-ios",
	"linux",
	"rumprun-netbsd",
	"unknown-freebsd",
	"unknown-illumos",
	];

... but that's another issue (later split into #3783), i.e. we really should not hardcode target triples in our artifact, as the complete list changes quite regularly.

The old component_(add|remove)() functions rely on the new_with_target() function; on the other hand, the new implementation of those two functions in this PR is based on distributable and fallback_target, which relies on detecting the component names (fetched from the distributable manifest) first, so will be more robust implementation-wise.

For the case in #3166:

rustup component remove rust-std-wasm32-unknown-unknown

Because of rustup cannot parse wasm32-unknown-unknown as a valid target, the rust-std-wasm32-unknown-unknown here is treated as a component name, and the target inferred to be x86_64-apple-darwin for this case. The error message as following:

error: toolchain 'nightly-x86_64-apple-darwin' does not contain component 'rust-std-wasm32-unknown-unknown' for target 'x86_64-apple-darwin'
note: not all platforms have the standard library pre-compiled: https://doc.rust-lang.org/nightly/rustc/platform-support.html

The actual component and target should be rust-std and wasm32-unknown-unknown in this error message respectively.
This is subtle because component name may also contains a '-' character (rust-std in this case), the component name is determined by user input subtract from possible target suffix currently (TryFrom<PartialTargetTriple>).

#3467 (comment)

[rami3l]

LGTM, and really sorry for the long wait! I personally would like to get this into Rustup v1.28.0.

[rami3l]

Nice work :)

[rami3l]

Thanks a lot! I think this PR is good to go.

[rami3l]

@djc @rbtcollins I guess we can ship this with v1.27.1 as well? As I see it, only rustup component (add|remove) is affected here and it's rather unlikely for this PR to introduce a regression somewhere given all our existing tests combined with the new ones...