fix(install): fix parse_npm_specifier for subpath specifiers
The hand-rolled parser returned the full "express/foo" / "@scope/pkg/
subpath" string when an npm specifier carried a subpath, which then
produced bogus tsconfig `paths` entries (e.g. "npm:express/foo" →
"./node_modules/express/foo") that wouldn't match the user's actual
`import "npm:express"` and pointed at a directory that didn't exist.
Delegate to deno_semver's NpmPackageReqReference, which already
correctly parses subpaths, scoped/unscoped names, and version
requirements, and pull just the package name out.
