embed: file names that start with underscore are not added to FS

[geokb]

It seems that files that start with underscore (e.g. _helpers.tpl) are not added to embed.FS variables. Thankfully for my own use case, I could just rename the file but this is honestly a very weird and unacceptable behavior for the majority of use cases.

[seankhliao]

Duplicate of #43854

[linquize]

filename starting with hyphen, such as -1.txt is also not added in go 1.16

pattern a/-1.txt: cannot embed file a/-1.txt: invalid name -1.txt